DOC-1752: Document Footnotes Plugin

[MitchC1999]

Related Ticket: DOC-1752

Description of Changes:

• Added footnotes plugin page and relevant partials
• Added references to the page in correct places

Need to do:

• [ ] Add demo page and include in footnotes.adoc
• [ ] Add release note (separate PR)
• [ ] Add a note about classes and CSS (like tableofcontents)
• [ ] Provide more accurate descriptions, more detail, and improve quality of writing

Pre-checks:

• [x] Branch prefixed with feature/6/ or hotfix/6/
• [x] modules/ROOT/nav.adoc has been updated (if applicable)
• [x] Files has been included where required (if applicable)
• [x] ~Files removed have been deleted, not just excluded from the build (if applicable)~
• [ ] (New product features only) Release Note added

Review:

• [ ] Documentation Team Lead has reviewed