Project dropdown in header/split up sidebars

[Machine-Maker]

Instead of having 1 header with dropdowns for each project and each project's categories, have 1 project dropdown on the left to switch between Paper, Velocity, Folia, and Waterfall. Then the buttons in the header can link to each category of documentation, Administration, Development, Contributing.

Then the sidebars for each of those categories can be simplified to only show docs in those categories instead of showing all the docs for paper, or all of velocity which it currently does.

I feel that most people aren't looking at administration and development docs for a project at the same time, so splitting them up unclutters the sidebars.