docs: Update lower bound on Sphinx to v5.1.1

[matthewfeickert]

Description

• Update lower bound of Sphinx to v5.1.1 to ensure consistent docs builds. This is simpler than disallowing particular problematic versions like Sphinx v5.1.0 given that the documentation can be viewed as a deployment of an application and does not need to support as wide of a version range as possible.
• Amends PR fix: Disallow Sphinx v5.1.0 to avoid 'exception: pop from an empty deque' #1925.

Checklist Before Requesting Reviewer

• Tests are passing
• "WIP" removed from the title of the pull request
• Selected an Assignee for the PR to be responsible for the log summary

Before Merging

For the PR Assignees:

• Summarize commit messages into a comprehensive review of the PR

* Update lower bound of Sphinx to v5.1.1 to ensure consistent docs
builds. This is simpler than disallowing particular problematic versions
like Sphinx v5.1.0 given that the documentation can be viewed as a
deployment of an application and does not need to support as wide of a
version range as possible.
* Amends PR #1925.

[matthewfeickert]

Should wait for PR #1919 to go in first and then rebase.

[matthewfeickert]

As this should be uncontroversial and getting this in should save time, I'm going to approve and merge this myself. As always, PRs approved by a single core dev can be reverted as needed by the rest of the dev team.