Adds finalized Knowledge Center Content

[reitermb]

This PR closes #1693

Summary of Changes

• Adds subdirectory to project updates environment to host Knowledge Center
• Adds Knowledge center content
• Bumps USWDS CSS version to support needed content
• Bumps jquery version
• Adds Lity to applicable pages to support accessible lightboxes
• Adds MVP version of "Back to top" functionality via anchor tags on applicable (read: long) pages

How to Test

This PR is identical to what's currently hosted in my Project Updates environment Staging Repo. It can be accessed at: https://reitermb.github.io/Test/knowledge-center/ and browsed as a repo: https://github.com/reitermb/Test

[codecov]

Codecov Report

Merging #1940 (79a4b86) into develop (7ba13f4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop    #1940   +/-   ##
========================================
  Coverage    95.17%   95.17%           
========================================
  Files           78       78           
  Lines         2072     2072           
  Branches       281      281           
========================================
  Hits          1972     1972           
  Misses          69       69           
  Partials        31       31           

Flag	Coverage Δ
dev-backend	95.80% <ø> (ø)
dev-frontend	93.84% <ø> (ø)

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[reitermb]

cc @ttran-hub @ADPennington
Re: Prior a11y review

• Sidenav List element has direct children that are not allowed inside <li> elements looks like it was a false positive
• All broken links should be working now
• All headings should be in the correct hierarchical order now
• Lengthy pages have a basic back to top link at the end now (there are ways to make it fancier, e.g. smooth scrolling and/or floating buttons in the future, but both would have added quite a bit of scope to do accessibly right now)
• Page titles should now be descriptive and unique
• Added what should be a fully accessible lightbox to all images (focus should be correctly trapped when the lightbox is open, alt text should be accessible by screenreaders in lightbox mode, keyboard nav should be supported for entering and exiting lightbox)

[ADPennington]

@ttran-hub this is ready for gov a11y review. @reitermb please connect with Thomas for a walk-through of the changes you mentioned 🆙

[ttran-hub]

Thanks @reitermb, the nav nesting fixes the list element under the first bullet one and others are good. a11y LGTM! 🚀

[ADPennington]

👍🏾 thanks @reitermb @lilybl1. cc: @ttran-hub @lfrohlich