docgen: implement cross-document links

[a-mr]

This proposal fully implements nim-lang/RFCs#125 Follow-up of: #18642 (for internal links) and #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 nim-lang/RFCs#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 docing a few files (after all indexes have been generated), and everywhere difference was negligible.
E.g. for lib/std/private/osfiles.nim that importdocs 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
3. 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

[Varriount]

@a-mr Any further work required for this?

[a-mr]

@Varriount
No, that was the last (mostly cosmetic) change I wanted to add.

[github-actions]

Thanks for your hard work on this PR!
The lines below are statistics of the Nim compiler built from 2620da9

Hint: mm: orc; opt: speed; options: -d:release
166134 lines; 8.488s; 611.559MiB peakmem