Add doc for IntelliJ

[kinow]

Hi,

I used the LSP in the past to test IntelliJ with Cylc ^1, so decided to try Benten + IntelliJ. It worked, so I added a page like the one from vim with instructions so others can replicate it in their IntelliJ environment :+1:

Thanks Bruno

[kghose]

Hi @kinow this looks awesome. I'm no longer involved with this project. Perhaps someone from SBG like @bogdang989 could take a look?

[kinow]

Moved to https://github.com/deNBI/denbi-benten/pull/22