Adjust field set usage docs sidebar links

[djptek]

• Have you signed the contributor license agreement? Y
• Have you followed the contributor guidelines? Y
• For proposing substantial changes or additions to the schema, have you reviewed the RFC process? n/a
• If submitting code/script changes, have you verified all tests pass locally using make test? Y
• If submitting schema/fields updates, have you generated new artifacts by running make and committed those changes? Y
• Is your pull request against master? Unless there is a good reason otherwise, we prefer pull requests against master and will backport as needed. Y
• Have you added an entry to the CHANGELOG.next.md? Y

Address #1072 (comment)

[kgeller]

lgtm

[djptek]

@ebeahan split lines are now (re-)concatenated

[ebeahan]

Looks great!