Website MkDocs migration

[kattni]

Updates for migrating website to MkDocs.

Includes:

• Header now uses Material tabs.
• Removes dynamic font sizing.
• Compensates for lack of dynamic font sizing; explicit font sizes were designed to work with the default, and therefore were not accurate when dynamic font sizing was disabled.
• Other things I'm not remembering right now.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct