docs: Lock docs dependencies even more

[paulgessinger]

Sphinx 4.1.0 breaks our docs build again. See #875.

Also resolve one doxygen error in the process.

[codecov]

Codecov Report

Merging #876 (1a27c64) into main (96f3a3f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #876   +/-   ##
=======================================
  Coverage   48.68%   48.68%           
=======================================
  Files         331      331           
  Lines       17128    17128           
  Branches     8084     8084           
=======================================
  Hits         8338     8338           
  Misses       3092     3092           
  Partials     5698     5698           

Impacted Files	Coverage Δ
Core/include/Acts/Utilities/PolymorphicValue.hpp	68.62% <ø> (ø)

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[paulgessinger]

I'm overriding the missing approval here.

[acts-project-service]

The backport to develop/v9.1.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-develop/v9.1.x develop/v9.1.x
# Navigate to the new working tree
cd .worktrees/backport-develop/v9.1.x
# Create a new branch
git switch --create backport-876-to-develop/v9.1.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 9a38f3fc4dda9a3f72fb6423bef7ae2acf90eb69
# Push it to GitHub
git push --set-upstream origin backport-876-to-develop/v9.1.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-develop/v9.1.x

Then, create a pull request where the base branch is develop/v9.1.x and the compare/head branch is backport-876-to-develop/v9.1.x.