Add sections for missing services in services.rst

[FroggyFlox]

As brought up by @phillxnet in rockstor/rockstor-core#2721 (review), we have some services that do not have a dedicated section in services.rst. This should be remediated.

The reasons for that differ so let's look at it service-by-service:

• Replication: a proper documentation is still needed: see replication documentation #30
• Rockstor: we should be able to easily add a short description as this only adjusts the nginx config file.
• S.M.A.R.T.: we already have a nice write-up for it (https://rockstor.com/docs/howtos/smart.html?highlight=smart#configure-monitoring) so we can write a short sentence or two and link to that write-up.
• Shell In A Box: a write-up remains needed: Write end user Web-UI embeded system shell doc #374
• SNMP: would need a write-up as well

We can address the case for the Rockstor and S.M.A.R.T. services, but the rest would require more work and would thus fall out of the scope of this issue. In the meantime, however, we can add some placeholders (a sentence or two for a very brief description) so that we have a matching list of services between our docs and Rockstor's UI.

In the same effort of ensuring a match between the list of Services in our docs and those in Rocsktor's UI, we may also want to re-order them according to what is in Rockstor's UI: alphabetical order (by default).