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/docs.md

@@ -0,0 +1,38 @@
+The documentation consists of several documents:
+
+- | `Tutorial (part I) <tut1.html>`_
+  | The Nim tutorial part one deals with the basics.
+
+- | `Tutorial (part II) <tut2.html>`_
+  | The Nim tutorial part two deals with the advanced language constructs.
+
+- | `Tutorial (part III) <tut3.html>`_
+  | The Nim tutorial part three about Nim's macro system.
+
+- | `Language Manual <manual.html>`_
+  | The Nim manual is a draft that will evolve into a proper specification.
+
+- | `Library documentation <lib.html>`_
+  | This document describes Nim's standard library.
+
+- | `Compiler user guide <nimc.html>`_
+  | The user guide lists command line arguments, special features of the
+    compiler, etc.
+
+- | `Tools documentation <tools.html>`_
+  | Description of some tools that come with the standard distribution.
+
+- | `Memory management <mm.html>`_
+  | Additional documentation about Nim's memory management strategies
+  | and how to operate them in a realtime setting.
+
+- | `Source code filters <filters.html>`_
+  | The Nim compiler supports source code filters as a simple yet powerful
+    builtin templating system.
+
+- | `Internal documentation <intern.html>`_
+  | The internal documentation describes how the compiler is implemented. Read
+    this if you want to hack the compiler.
+
+- | `Index <theindex.html>`_
+  | The generated index. **Index + (Ctrl+F) == Joy**