Updating developer install guide docs

[Nicola Nye]

Hi Conrad (and anyone else with a development install),

With Ellie away, you're now my expert (!) on getting a developer
environment up and running. I'm looking at freshening the docs on our
official docs.cyrus.foundation site.

I know you have followed Ellie's install notes which have been
summarised at
https://git.cyrus.foundation/w/setup_a_cyrus_development_environment/
and had great success.

While I am tempted to just overwrite the original install guide with
Ellie's new instructions, could you just give it a glance through and
let me know if there's any detail in there that's worth retaining?
https://docs.cyrus.foundation/imap/installation/diy.html

I noted that the dependencies were described in greater detail along
with where to get them from (and the list seemed a little different)

The original also talks about a configure step which Ellie's guide
doesn't have. Should it still be referenced for completeness?

Cheers,

   Nicola