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

tools/kochdocs.nim

@@ -107,18 +107,18 @@ proc nimCompileFold*(desc, input: string, outputDir = "bin", mode = "c", options
   let cmd = findNim().quoteShell() & " " & mode & " -o:" & output & " " & options & " " & input
   execFold(desc, cmd)
 
-proc getRst2html(): seq[string] =
+proc getMd2html(): seq[string] =
   for a in walkDirRecFilter("doc"):
     let path = a.path
-    if a.kind == pcFile and path.splitFile.ext == ".rst" and path.lastPathPart notin
-        ["docs.rst", "nimfix.rst",
-         "docstyle.rst" # docstyle.rst shouldn't be converted to html separately;
-                        # it's included in contributing.rst.
+    if a.kind == pcFile and path.splitFile.ext == ".md" and path.lastPathPart notin
+        ["docs.md", "nimfix.md",
+         "docstyle.md" # docstyle.md shouldn't be converted to html separately;
+                       # it's included in contributing.md.
         ]:
           # maybe we should still show nimfix, could help reviving it
           # `docs` is redundant with `overview`, might as well remove that file?
       result.add path
-  doAssert "doc/manual/var_t_return.rst".unixToNativePath in result # sanity check
+  doAssert "doc/manual/var_t_return.md".unixToNativePath in result # sanity check
 
 const
   rstPdfList = """
@@ -253,13 +253,13 @@ proc buildDocPackages(nimArgs, destPath: string) =
 
 proc buildDoc(nimArgs, destPath: string) =
   # call nim for the documentation:
-  let rst2html = getRst2html()
+  let rst2html = getMd2html()
   var
     commands = newSeq[string](rst2html.len + len(doc0) + len(doc) + withoutIndex.len)
     i = 0
   let nim = findNim().quoteShell()
   for d in items(rst2html):
-    commands[i] = nim & " rst2html $# --git.url:$# -o:$# --index:on $#" %
+    commands[i] = nim & " md2html $# --git.url:$# -o:$# --index:on $#" %
       [nimArgs, gitUrl,
       destPath / changeFileExt(splitFile(d).name, "html"), d]
     i.inc