Improve Markdown code blocks & start moving docs to Markdown style

[a-mr]

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

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

  then in Markdown that will be:

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

• 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 interprets it as quoted block, for Markdown it's just normal paragraphs. Fixes bug 23 of RST minor bugs #17340.

• fix indentation inside Markdown code blocks — additional indentation is preserved there — bug 22 of RST minor bugs #17340

• add separate md2html and md2tex commands. This is to separate Markdown behavior in cases when it diverges w.r.t. RST significantly — most conspicuously 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

• allow more than 3 back-ticks open/close blocks (tildes ~ are still not allowed to avoid conflict with RST adornment headings) see also universal inline code block syntax for nim doc+rst RFCs#355

• better error messages

• (other) fix a bug that admonitions cannot be used in sandbox mode; fix annoying warning on line 2711

Current rendering of .md files in Github is far from being optimal, see e.g. Manual: https://github.com/a-mr/Nim/blob/markdown-better-code-blocks/doc/manual.md, which is partially because we use ~~~ as heading adornments but they are interpreted as code blocks in Markdown.

[a-mr]

~~~ headings will be changed to ^^^ ones in follow up PRs.

[a-mr]

Ping @Araq

[Araq]

Fyi even heavy refactorings like "reimplemented the markdown parser and it's not based anymore on the RST parser" will be accepted. RST is dead, Markdown has won.

[Araq]

Also, if you tell me your personal information via e-mail I'll send you a signed copy of "Mastering Nim" as a reward for your outstanding work on Nim's doc gen.

[github-actions]

Thanks for your hard work on this PR!
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