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/nimgrep.rst

@@ -1,69 +0,0 @@
-=========================
-  nimgrep User's manual
-=========================
-
-:Author: Andreas Rumpf
-:Version: 1.6.0
-
-.. default-role:: option
-.. contents::
-
-Nimgrep is a command line tool for search and replace tasks. It can search for
-regex or peg patterns and can search whole directories at once. User
-confirmation for every single replace operation can be requested.
-
-Nimgrep has particularly good support for Nim's
-eccentric *style insensitivity* (see option `-y` below).
-Apart from that it is a generic text manipulation tool.
-
-
-Installation
-============
-
-Compile nimgrep with the command:
-
-.. code:: cmd
-  nim c -d:release tools/nimgrep.nim
-
-And copy the executable somewhere in your ``$PATH``.
-
-
-Command line switches
-=====================
-
-.. include:: nimgrep_cmdline.txt
-
-Examples
-========
-
-All examples below use default PCRE Regex patterns:
-
-+ To search recursively in Nim files using style-insensitive identifiers:
-
-  .. code:: cmd
-    nimgrep --recursive --ext:'nim|nims' --ignoreStyle
-    # short: -r --ext:'nim|nims' -y
-
-  .. Note:: we used `'` quotes to avoid special treatment of `|` symbol
-    for shells like Bash
-
-+ To exclude version control directories (Git, Mercurial=hg, Subversion=svn)
-  from the search:
-
-  .. code:: cmd
-    nimgrep --excludeDir:'^\.git$' --excludeDir:'^\.hg$' --excludeDir:'^\.svn$'
-    # short: --ed:'^\.git$' --ed:'^\.hg$' --ed:'^\.svn$'
-
-+ To search only in paths containing the `tests` sub-directory recursively::
-
-  .. code:: cmd
-    nimgrep --recursive --includeDir:'(^|/)tests($|/)'
-    # short: -r --id:'(^|/)tests($|/)'
-
-  .. Attention:: note the subtle difference between `--excludeDir`:option: and
-    `--includeDir`:option:\: the former is applied to relative directory entries
-    and the latter is applied to the whole paths
-
-+ Nimgrep can search multi-line, e.g. to find files containing `import`
-  and then `strutils` use pattern `'import(.|\n)*?strutils'`:option:.
-