Add content library

[drewnielson]

Content Library includes the following features and functions:

• Dynamic filtering of "library cards" across multiple filters, including topic, sub-topic, audience, resource type, and format
• And/Or logic for additive (or) filters within a filter type/category and reductive (and) filters between filter types/categories
• Active filters list pill boxes with the ability to remove individual filters from the filter selection accordions or from the active filters list pill boxes
• Filters reset to remove all filters at once
• URL parameters for each filter, to make it possible to bookmark and share page link with filters already selected
• Dynamic disabling of filters when filters become unavailable (or wouldn't make any difference if selected) based on the current filter selection

Still to do:

• Fix front matter inconsistencies, including misspellings, duplicate/alternate filters, pages with missing or bad filter parameters in front matter, add library-exclude: true to certain additional pages
• Devise method for incorporating documents in assets folders that do not have a page with front matter
• Consider possible color schemes for library card categories/filters
• Consider pagination to reduce scrolling
• Consider keyboard shortcuts to specific page sections
• Consider adding short page/document descriptions to library cards in accordion, help text, or other expanding content
• Add highlights content/announcement for home page