Add docs for Semaphore

[armanbilge]

Fixes #1927 mostly by way of CE2 copypasta.

[djspiewak]

Very nice!!! Thank you!

Would you mind adding this to the sidebar.json file? That will make it show up on the site.

[armanbilge]

Np, this was an easy one :) fixed the sidebar, and also noticed the sidebar TODO. Still quite a bit to do? Adding a page for IO would be nice, the lack of docs for IOLocal came up on Discord but I wasn't sure where they should go.

[djspiewak]

Np, this was an easy one :) fixed the sidebar, and also noticed the sidebar TODO. Still quite a bit to do? Adding a page for IO would be nice, the lack of docs for IOLocal came up on Discord but I wasn't sure where they should go.

Not sure either; let's just write them first and then place them somewhere! :-)

LOTS of stuff to do. I have a somewhat outdated list in #1715, and more generally in the 📚 docs label