Docs: fix broken internal refs, fix broken intersphinx cross-refs to multidict, minor styling fixes

[hoefling]

What do these changes do?

This PR contains only documentation improvements, no code changes.

1. Broken internal refs to e.g. URL.human_repr() are fixed.
2. Broken crossrefs are fixed: dropped duplicate intersphinx config, fixed broken URL to multidict docs.
3. Markdown styling that hasn't the same meaning in ReST fixed.

Are there changes in behavior for the user?

No

Related issue number

Didn't create one since the changes are trivial IMO

Checklist

• I think the code is well written
• Unit tests for the changes exist not needed, but an extra job in the ci action could watch over the docs and fail if new errors are introduced
• Documentation reflects the changes
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

[hoefling]

Docs built from this PR

[codecov]

Codecov Report

Merging #665 (b5a0ab2) into master (2830a3d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #665   +/-   ##
=======================================
  Coverage   99.72%   99.72%           
=======================================
  Files           4        4           
  Lines         717      717           
  Branches      162      162           
=======================================
  Hits          715      715           
  Misses          2        2           

Flag	Coverage Δ
unit	99.58% <ø> (ø)

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[webknjaz]

Looks like this patch could be coupled with adding strictness to the Sphinx setup. I like having default_role = 'any' and nitpicky = True in conf.py for this.
Also, adding -W --keep-going -n to the sphinx-build invocation is as helpful.

[asvetlov]

LGTM, thanks!