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/nimfix.rst

@@ -1,58 +0,0 @@
-.. default-role:: code
-
-=====================
-  Nimfix User Guide
-=====================
-
-:Author: Andreas Rumpf
-:Version: |nimversion|
-
-**WARNING**: Nimfix is currently beta-quality.
-
-Nimfix is a tool to help you upgrade from Nimrod (<= version 0.9.6) to
-Nim (=> version 0.10.0).
-
-It performs 3 different actions:
-
-1. It makes your code case consistent.
-2. It renames every symbol that has a deprecation rule. So if a module has a
-   rule `{.deprecated: [TFoo: Foo].}` then `TFoo` is replaced by `Foo`.
-3. It can also check that your identifiers adhere to the official style guide
-   and optionally modify them to do so (via `--styleCheck:auto`).
-
-Note that `nimfix` defaults to **overwrite** your code unless you
-use `--overwriteFiles:off`! But hey, if you do not use a version control
-system by this day and age, your project is already in big trouble.
-
-
-Installation
-------------
-
-Nimfix is part of the compiler distribution. Compile via::
-
-  nim c compiler/nimfix/nimfix.nim
-  mv compiler/nimfix/nimfix bin
-
-Or on windows::
-
-  nim c compiler\nimfix\nimfix.nim
-  move compiler\nimfix\nimfix.exe bin
-
-Usage
------
-
-Usage:
-  nimfix [options] projectfile.nim
-
-Options:
-
-  --overwriteFiles:on|off       overwrite the original nim files. DEFAULT is ON!
-  --wholeProject                overwrite every processed file.
-  --checkExtern:on|off          style check also extern names
-  --styleCheck:on|off|auto      performs style checking for identifiers
-                                and suggests an alternative spelling;
-                                'auto' corrects the spelling.
-
-In addition, all command line options of Nim are supported.
-
-