docgen: implement cross-document links (#20990)

* docgen: implement cross-document links

Fully implements https://github.com/nim-lang/RFCs/issues/125
Follow-up of: https://github.com/nim-lang/Nim/pull/18642 (for internal links)
and https://github.com/nim-lang/Nim/issues/20127.

Overview
--------

Explicit import-like directive is required, called `.. importdoc::`.
(the syntax is % RST, Markdown will use it for a while).

Then one can reference any symbols/headings/anchors, as if they
were in the local file (but they will be prefixed with a module name
or markup document in link text).
It's possible to reference anything from anywhere (any direction
in `.nim`/`.md`/`.rst` files).

See `doc/docgen.md` for full description.

Working is based on `.idx` files, hence one needs to generate
all `.idx` beforehand. A dedicated option `--index:only` is introduced
(and a separate stage for `--index:only` is added to `kochdocs.nim`).

Performance note
----------------

Full run for `./koch docs` now takes 185% of the time before this PR.
(After: 315 s, before: 170 s on my PC).
All the time seems to be spent on `--index:only` run, which takes
almost as much (85%) of normal doc run -- it seems that most time
is spent on file parsing, turning off HTML generation phase has not
helped much.
(One could avoid it by specifying list of files that can be referenced
and pre-processing only them. But it can become error-prone and I assume
that these linke will be **everywhere** in the repository anyway,
especially considering https://github.com/nim-lang/RFCs/issues/478.
So every `.nim`/`.md` file is processed for `.idx` first).

But that's all without significant part of repository converted to
cross-module auto links. To estimate impact I checked the time for
`doc`ing a few files (after all indexes have been generated), and
everywhere difference was **negligible**.
E.g. for `lib/std/private/osfiles.nim` that `importdoc`s large
`os.idx` and hence should have been a case with relatively large
performance impact, but:

* After: 0.59 s.
* Before: 0.59 s.

So Nim compiler works so slow that doc part basically does not matter :-)

Testing
-------

1) added `extlinks` test to `nimdoc/`
2) checked that `theindex.html` is still correct
2) fixed broken auto-links for modules that were derived from `os.nim`
   by adding appropriate ``importdoc``

Implementation note
-------------------

Parsing and formating of `.idx` entries is moved into a dedicated
`rstidx.nim` module from `rstgen.nim`.

`.idx` file format changed:

* fields are not escaped in most cases because we need original
  strings for referencing, not HTML ones
  (the exception is linkTitle for titles and headings).
  Escaping happens later -- on the stage of `rstgen` buildIndex, etc.
* all lines have fixed number of columns 6
* added discriminator tag as a first column,
  it always allows distinguish Nim/markup entries, titles/headings, etc.
  `rstgen` does not rely any more (in most cases) on ad-hoc logic
  to determine what type each entry is.
* there is now always a title entry added at the first line.
* add a line number as 6th column
* linkTitle (4th) column has a different format: before it was like
  `module: funcName()`, now it's `proc funcName()`.
  (This format is also propagated to `theindex.html` and search results,
  I kept it that way since I like it more though it's discussible.)
  This column is what used for Nim symbols resolution.
* also changed details on column format for headings and titles:
  "keyword" is original, "linkTitle" is HTML one

* fix paths on Windows + more clear code

* Update compiler/docgen.nim

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

* Handle .md and .nim paths uniformly in findRefFile

* handle titles better + more comments

* don't allow markup overwrite index title for .nim files

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

2 files changed, 3 insertions, 2 deletions

diff --git a/nimdoc/test_out_index_dot_html/expected/foo.idx b/nimdoc/test_out_index_dot_html/expected/foo.idx
index a8dabb67e..ac76aa532 100644
--- a/nimdoc/test_out_index_dot_html/expected/foo.idx
+++ b/nimdoc/test_out_index_dot_html/expected/foo.idx
@@ -1 +1,2 @@
-foo	index.html#foo	foo: foo()	
+nimTitle	foo	index.html	module nimdoc/test_out_index_dot_html/foo		0
+nim	foo	index.html#foo	proc foo()		1
diff --git a/nimdoc/test_out_index_dot_html/expected/theindex.html b/nimdoc/test_out_index_dot_html/expected/theindex.html
index 2a0ea7900..ea3486d4b 100644
--- a/nimdoc/test_out_index_dot_html/expected/theindex.html
+++ b/nimdoc/test_out_index_dot_html/expected/theindex.html
@@ -24,7 +24,7 @@
       Modules: <a href="index.html">index</a>.<br/><p /><h2>API symbols</h2>
 <dl><dt><a name="foo" href="#foo"><span>foo:</span></a></dt><dd><ul class="simple">
 <li><a class="reference external"
-          data-doc-search-tag="foo: foo()" href="index.html#foo">foo: foo()</a></li>
+          data-doc-search-tag="foo: proc foo()" href="index.html#foo">foo: proc foo()</a></li>
           </ul></dd>
 </dl>
       <div class="twelve-columns footer">