Bump sphinx-lint to 0.6.8

[hugovk]

Documentation

Bump sphinx-lint to 0.6.8 in .pre-commit-config.yaml and fix any newly reported issues.

https://github.com/sphinx-contrib/sphinx-lint/releases/tag/v0.6.8

Linked PRs

• gh-106978: Bump sphinx-lint to 0.6.8 #106990
• gh-106978: Bump sphinx-lint to 0.6.8 #106991
• [3.12] Bump sphinx-lint to 0.6.8 (gh-106978) #107023

[bedevere-bot]

GH-107023 is a backport of this pull request to the 3.12 branch.