Add Ubuntu 20.04 install and migration instructions

[eloquence]

As part of the Ubuntu 20.04 transition (freedomofpress/securedrop#4768) we need to update our installation guide to be accurate for Ubuntu 20.04. We also need to provide migration instructions for administrators. For the latter, we may be able to build on Ro's generalized migration docs in #133.

[eloquence]

Last time around we also had a "xenial prep" doc (freedomofpress/securedrop#4046); will create similar instructions specifically mentioning v3 as a preparatory step folks can complete pre-Focal, towards this issue.

[eloquence]

@zenmonkeykstop has offered to sketch out the migration process as part of the current sprint, informed by the generalized migration procedures @rocodes wrote up in #133. This can be done in a wiki page for now; our initial goal is to ensure that we have a migration procedure we can test for the following configurations:

• v2-only (v2 onion service config will not be restored once Disable v2 onion addresses on restore on Focal securedrop#5677 is merged)
• v3+v2 or v3-only

[eloquence]

Completed mainly via #169, #168 and #154.