A variety of website updates

[david-waltermire]

Committer Notes

This PR contains the following updates:

• Refactoring of model documentation to group models by layer.
• Created a new "tools" page with a link in the main navigation.
• Added ICS file to Lunch with the Devs; also posted slides from the last two meetings.
• Closes Search bar misaligned with top-right nav links on website #443.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you squashed any non-relevant commits and commit messages? [instructions]
• Do all automated CI/CD checks pass?

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you included examples of how to use your new feature(s)?
• Have you updated all OSCAL website and readme documentation affected by the changes you made? Changes to the OSCAL website can be made in the docs/content directory of your branch.

[wendellpiez]

Not essential but nice: put the left nav / ToC into the same order as the diagram on the /documentation/schema page.

In general, it looks great --

[david-waltermire]

Addressed review feedback.