Add link check workflow and fix broken links

[stephprince]

Summary of changes

• fix #558, #553
• update broken links
• add GitHub workflow for sphinx link checks using nitpicky mode

Checklist

For all schema changes:

• [ ] Add release notes for the PR to docs/format/source/format_release_notes.rst.

If this is the first schema change after a schema release (i.e., the version string in core/nwb.namespace.yaml does not end in "-alpha"), then:

• [ ] Update the version string in core/nwb.namespace.yaml and core/nwb.file.yaml to the next major/minor/patch version with the suffix "-alpha". For example, if the current version is 2.4.0 and this is a minor change, then the new version string should be "2.5.0-alpha".
• [ ] Update the value of the version variable in docs/format/source/conf.py to the next version without the suffix "-alpha", e.g., "2.5.0".
• [ ] Update the value of the release variable in docs/format/source/conf.py to the next version with the suffix "-alpha", e.g., "2.5.0-alpha".
• [ ] Add a new section in the release notes docs/format/source/format_release_notes.rst for the new version with the date "Upcoming" in parentheses.

[rly]

Thank you @stephprince ! This looks good. I would like to merge #560 first which addresses unrelated repo issues.