Fixes #3960: Add details in the database maintenance chapter about how a...

...rchival/deletion works

Ticket: https://www.rudder-project.org/redmine/issues/3960

To be merged in 2.6
I don't understand these 2 metrics - they seem to be the same. If they are, please clarify that: they shouldn't be in bullet points, you could just make a sentence like:

> The estimated disk space consumption, with a 5 minute agent run frequency, is 150 to 400 kB per Directive, per day and per node, which is roughly 5 to 10 MB per Directive per month and per node.