Add RST style checker (doc8)

[DelazJ]

refs #10856

Note: doc8 seems not to recognize :index: role which is a valid role. Adjusted the text to avoid their use but...

[timlinux]

LGTM

[qgis-bot]

The backport to release_3.44 failed:

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

stderr

error: could not apply 844ab89147... Add doc8 checker
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config set advice.mergeConflict false"

stdout

Auto-merging .pre-commit-config.yaml
CONFLICT (content): Merge conflict in .pre-commit-config.yaml

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-release_3.44 release_3.44
# Navigate to the new working tree
cd .worktrees/backport-release_3.44
# Create a new branch
git switch --create backport-10903-to-release_3.44
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 844ab89147792cbf06131b868da4ad1a75d796af,ce82240f34ab6394bd5ec7f26196c69ea390235a,8b6f27a37ce38a4a2f84902c276151507db946a7
# Push it to GitHub
git push --set-upstream origin backport-10903-to-release_3.44
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release_3.44

Then, create a pull request where the base branch is release_3.44 and the compare/head branch is backport-10903-to-release_3.44.