doc: add instructions for compiling cephadm

doc: add instructions for compiling cephadm

Now that cephadm is based on zipapp, add a short section to the
developer docs explaining how to build cephadm yourself.

Note: This commit is a cherry-pick of
9ad38033cc5c7f177cb8fe3bae696682687e0346, which was introduced by John
Mulligan in #48180. This is one of three commits introduced in that PR,
and this cherry-pick cleans up omissions I (Zac Dover) inadvertently
introduced while attempting to rectify the merge conflicts in #51843. I
expect that one more cherry-picked commit (specifically,
d11cf0e82aab8d4cef9d423e5d463a373eaf383a, which cannot be merged easily
until d7921e88d69b4bc355da9c0327cc33e59e7d7abb has been merged into
main, for reasons that are too
Rick-and-Morty-there-should-never-be-more-than-one-dot to go into here)
will follow this one.

Signed-off-by: John Mulligan <jmulligan@redhat.com>
(cherry picked from commit 9ad38033cc5c7f177cb8fe3bae696682687e0346)