Update docs for Capital Framework v7.3.0 + other hygiene

[Scotchester]

Also includes some GitHub Pages hygiene updates.

Additions

• Restores the copying of the icon font files into the static folder until we can totally kill it dead

Removals

• Several HTML and markdown files that were no longer used

Changes

• Updates usage files with the latest updates from Capital Framework
• Updates all icons that aren't part of the legacy cf-icons docs to be SVG
• Pulled out common top- and bottom-of-page stuff into page-top.html and page-bottom.html includes, so that that stuff isn't duplicated between the default.html and home.html layouts
• Replaced usage of site.github.url with the relative_url filter per the latest guidance in Jekyll's docs
• Converts {% highlight %} tags to GFM-style fenced code blocks

Testing

1. Pull branch
2. npm run cf-unlink, in case you have any local dev links still in place
3. npm install
4. gulp
5. bundle update
6. bundle exec jekyll serve watch
7. Load up http://localhost:4000/ and surf around; make sure nothing looks broken

Notes

• Found one other extant use of the icon font in cf-core's usage.md file (the .u-visually-hidden example, but that needs to be taken care of upstream in the source file on canary/master.

Checklist

• [ ] PR has an informative and human-readable title
• [ ] Changes are limited to a single goal (no scope creep)
• [ ] Code can be automatically merged (no conflicts)
• [ ] Code follows the standards laid out in the development playbook
• [ ] Passes all existing automated tests
• [ ] Any change in functionality is tested
• [ ] New functions are documented (with a description, list of inputs, and expected output)
• [ ] Placeholder code is flagged / future todos are captured in comments
• [ ] Visually tested in supported browsers and devices (see checklist below :point_down:)
• [ ] Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• [ ] Reviewers requested with the Reviewers tool :arrow_right:

[Scotchester]

Those Snyk errors are expected; this branch doesn't have those files in it.

[anselmbradford]

Some things you might want to fix while you're in here:

• "Compiled source" in sidebar on http://localhost:4000/getting-started/ is broken.
• http://localhost:4000/getting-started/#using-the-generator links to Grunt, but that has been removed from the generator (aside: generator-cf Readme still references Grunt).
• http://localhost:4000/components/cf-atomic-component/ usage link references #Usage but the id is usage. Not sure if this is an issue, but it doesn't appear the link works.
• http://localhost:4000/contributing/#automatically-create-an-alternate-stylesheet-for-older-ie has instructions for grunt-legacssy, which we don't use.
• http://localhost:4000/about/#browser-support--testing mentions Blackberry Bold, which can be removed.

[Scotchester]

@anselmbradford Thanks for the helpful review!

1. Broken compiled source links: As noted in https://github.com/cfpb/capital-framework/pull/846#issuecomment-439160938, looks like this has been broken since 7.0.0. We need to put the .npmignore back with an explanation that it's required for npm to not ignore that dist folder, which needs to be done in a canary/master PR and a new release.
2. Usage TOC link error: This also needs to be updated in the component's source usage.md file.

Other changes have been made!