walkDirRecFilter, update doc CI filter, compiler/index.nim for docs + various other fixes

[timotheecour]

update doc CI filter to include the files mostly likely to require doc rebuild

unlike previous attempt (https://github.com/nim-lang/Nim/pull/13853/files), this is much more targeted, just the usual suspects that affect docs the most.

• not sure what's the need for special casing doc/nimdoc.css ; ok to just merge those 2 lists to avoid duplication (in this or a cleanup pr)?
• I've also asked here https://github.community/t/how-to-factor-paths-in-common-for-push-and-pull-request/115967 for how to refactor 2 paths lists to avoid duplication by having a common subset

EDIT: CI fails on windows, which is a reason why this PR is needed (catch issues on code PRs that introduce problem, not later); I suspect it's because nim doc --project compiler/nim fails on windows... investigating

refactor all workaround implementations of nativeToUnixPath into 1

(I still need to finish #13265)

added compiler/index.nim

because https://nim-lang.github.io/Nim/compiler/nim.html was very underwhelming
after this PR navigating to https://nim-lang.github.io/Nim/compiler/ will do something useful

removed template duplication in config/nimdoc.cfg

and made sure it all works; so there's only 1 template to update

added undocumented ./koch --docslocal:myhtmldocs

so you can build all the docs n 40s instead of 190s as follows:
./koch --nim:$nimb --docslocal:htmldocs2 --doccmd:skip --warnings:off --hints:off
which is useful during local development when you need to run this a few times

added lib/std/private/globs.nim

for internal use only only (ie for any module inside nim repo) for now, but aim is to eventually merge those into stdlib.os once API stabilizes; same spirit as compiler/pathutils except lib/std/private/ is the perfect staging place IMO if you need to use it from not just compiler/ but other places (eg kochdocs, koch, tools, compiler, other modules in stdlib etc), giving it some internal exposure to give a chance to fix API without breaking user code, before converting it.

private conveys exactly what it is: for internal use only, if you use it in your code outside of nim repo it may break at any time

• added walkDirRecFilter
  that's IMO the better API, and the most flexible (and walkDirRec can later be implemented on top of it)
  It's a common requested feature (eg see recent thread Can I "prune" directories with walkDirRect? - Nim forum) and should be in stdlib; but I also needed it in this PR, see below

• fix a bug in getDocList
  this was picking up all the nim files in various nimcache folders in my repo (eg those created by runnableExamples); noone noticed since these don't export any symbols and therefore don't end up in the docs, but nim doc was being run on all of these, and there were quite a lot, slowing down koch docs.

The clean way to filter is via a follow filter (ie "filter early" before recursing down a folder instead of filtering at the end which is less efficient), hence walkDirRecFilter

fix a bug in lib/std/time_t.nim

which was making docs CI fail on windows since the compiler docs were enabled

used a glob to generate the rst files; added back docstyle which wasn't being docgen'd

i added docstyle which seemed like an unintentional omission
I kept docs.rst out since it's just an include
ditto with nimfix which is dead for now ( :-( ), but I wouldn't mind showing it in docs, would increase likelihood it gets revived

=> that's why glob+blacklist is always better than whitelist