Improve Nav Mgmt. Via `mkdocs-awesome-pages-plugin`

[wesleyboar]

Proposal

Install mkdocs-awesome-pages-plugin so complex custom navigation is easier to manage (especially for https://github.com/DesignSafe-CI/DS-User-Guide).

Why

• To replace giant pages using include-markdown with small pages and .pages.
• To move navigation management from root mkdocs.yml to relevant directory.
• (Maybe) To fix the problem of search showing duplicate results.

Related

• required by Make Navigation Easier to Manage DesignSafe-CI/DS-User-Guide#94