Add info about visa and relocation costs to FAQ

[yongrenjie]

Summary

As title. These are obviously really useful things for potential applicants to know, and the fact that the Turing sponsors visas is quite a big deal for non-UK nationals :)

Supersedes https://github.com/alan-turing-institute/Hut23/issues/1667

Todo @yongrenjie

• write about visa
• find someone to write about relocation costs
• write about equipment (laptop, monitor, etc)

[penelopeysm]

Related to #154, but it's probably a good idea to have a separate page on this too. Will work on it.