restructure

[karenmajewicz]

This PR will migrate all of the docs into the main GBL website. Things to consider:

• The migrated pages are all in a directory called "documentation". This differentiates them from the default directory of "docs". We could choose to organize them differently
• The layout of the menu could be adjusted to combine or split sections

[karenmajewicz]

[karenmajewicz]

• [x] Will need to be rebased after merging #231