Docs: move permalinks from documents into config

[parkr]

Reverts #5532 in favor of a global permalink 😄

[parkr]

/cc @jekyll/documentation

[parkr]

script/proof with --disable-external:

- ./_site/docs/maintaining/becoming-a-maintainer/index.html
  *  internally linking to reviewing-a-pull-request.md, which does not exist (line 311)
  *  internally linking to triaging-an-issue.md, which does not exist (line 299)
- ./_site/docs/maintaining/merging-a-pull-request/index.html
  *  internally linking to reviewing-a-pull-request.md, which does not exist (line 295)
- ./_site/docs/maintaining/reviewing-a-pull-request/index.html
  *  internally linking to ../CONDUCT.markdown, which does not exist (line 293)
- ./_site/docs/maintaining/special-labels/index.html
  *  internally linking to triaging-an-issue.md#staleness-and-automatic-closure, which does not exist (line 305)
  *  trying to find hash of triaging-an-issue.md#staleness-and-automatic-closure, but /Users/parkr/jekyll/jekyll/_site/docs/maintaining/special-labels/index.html/triaging-an-issue.md does not exist (line 305)
htmlproof 2.6.4 | Error:  HTML-Proofer found 6 failures!

Gonna fix those here too.

[parkr]

Welp. 7af4164

[zlatanvasovic]

Seems fine to me. Just gotta resolve merge issues.

[parkr]

oh @DirtyF did you already fix these?

[DirtyF]

@parkr yes, everything was OK yesterday, the remaining conflict is just about a link I add in a page. Feel free to review it on your machine.

[DirtyF]

@jekyllbot: merge +site