Improve Markdown code blocks & start moving docs to Markdown style (#19954)

- add additional parameters parsing (other implementations will just
  ignore them). E.g. if in RST we have:

  .. code:: nim
     :test: "nim c $1"

     ...

  then in Markdown that will be:

  ```nim test="nim c $1"
  ...
  ```

- implement Markdown interpretation of additional indentation which is
  less than 4 spaces (>=4 spaces is a code block but it's not
implemented yet). RST interpretes it as quoted block, for Markdown it's
just normal paragraphs.
- add separate `md2html` and `md2tex` commands. This is to separate
  Markdown behavior in cases when it diverges w.r.t. RST significantly —
most conspicously like in the case of additional indentation above, and
also currently the contradicting inline rule of Markdown is also turned
on only in `md2html` and `md2tex`. **Rationale:** mixing Markdown and
RST arbitrarily is a way to nowhere, we need to provide a way to fix the
particular behavior. Note that still all commands have **both** Markdown
and RST features **enabled**. In this PR `*.nim` files can be processed
only in Markdown mode, while `md2html` is for `*.md` files and
`rst2html` for `*.rst` files.
- rename `*.rst` files to `.*md` as our current default behavior is
  already Markdown-ish
- convert code blocks in `docgen.rst` to Markdown style as an example.
  Other code blocks will be converted in the follow-up PRs
- fix indentation inside Markdown code blocks — additional indentation
  is preserved there
- allow more than 3 backticks open/close blocks (tildas \~ are still not
  allowed to avoid conflict with RST adornment headings) see also
https://github.com/nim-lang/RFCs/issues/355
- better error messages
- (other) fix a bug that admonitions cannot be used in sandbox mode; fix
  annoying warning on line 2711

doc/apis.rst

@@ -1,85 +0,0 @@
-=================
-API naming design
-=================
-
-.. default-role:: code
-.. include:: rstcommon.rst
-
-The API is designed to be **easy to use** and consistent. Ease of use is
-measured by the number of calls to achieve a concrete high-level action.
-
-
-Naming scheme
-=============
-
-The library uses a simple naming scheme that makes use of common abbreviations
-to keep the names short but meaningful. Since version 0.8.2 many symbols have
-been renamed to fit this scheme. The ultimate goal is that the programmer can
-*guess* a name.
-
-
-===================     ============   ======================================
-English word            To use         Notes
-===================     ============   ======================================
-initialize              initT          `init` is used to create a
-                                       value type `T`
-new                     newP           `new` is used to create a
-                                       reference type `P`
-find                    find           should return the position where
-                                       something was found; for a bool result
-                                       use `contains`
-contains                contains       often short for `find() >= 0`
-append                  add            use `add` instead of `append`
-compare                 cmp            should return an int with the
-                                       `< 0` `== 0` or `> 0` semantics;
-                                       for a bool result use `sameXYZ`
-put                     put, `[]=`     consider overloading `[]=` for put
-get                     get, `[]`      consider overloading `[]` for get;
-                                       consider to not use `get` as a
-                                       prefix: `len` instead of `getLen`
-length                  len            also used for *number of elements*
-size                    size, len      size should refer to a byte size
-capacity                cap
-memory                  mem            implies a low-level operation
-items                   items          default iterator over a collection
-pairs                   pairs          iterator over (key, value) pairs
-delete                  delete, del    del is supposed to be faster than
-                                       delete, because it does not keep
-                                       the order; delete keeps the order
-remove                  delete, del    inconsistent right now
-remove-and-return       pop            `Table`/`TableRef` alias to `take`
-include                 incl
-exclude                 excl
-command                 cmd
-execute                 exec
-environment             env
-variable                var
-value                   value, val     val is preferred, inconsistent right
-                                       now
-executable              exe
-directory               dir
-path                    path           path is the string "/usr/bin" (for
-                                       example), dir is the content of
-                                       "/usr/bin"; inconsistent right now
-extension               ext
-separator               sep
-column                  col, column    col is preferred, inconsistent right
-                                       now
-application             app
-configuration           cfg
-message                 msg
-argument                arg
-object                  obj
-parameter               param
-operator                opr
-procedure               proc
-function                func
-coordinate              coord
-rectangle               rect
-point                   point
-symbol                  sym
-literal                 lit
-string                  str
-identifier              ident
-indentation             indent
-===================     ============   ======================================