[Docs] Update tunnelbroker configuration section

Summary:
Update tunnelbroker docs to reflect new state-of-the-world, in that
it requires a default keyserver now.

Add configuration files to .gitignore to avoid developers clobbering each
others dev environments.

Requirement added in https://phab.comm.dev/D2903

https://linear.app/comm/issue/ENG-1971

Depends on D5246

Test Plan: N/A

Reviewers: max, tomek, ashoat, varun

Reviewed By: max, ashoat

Subscribers: atul, abosh

Differential Revision: https://phab.comm.dev/D5304