feat(docs): Add documentation for the rewrite command

[s-leroux]

This adds documentation for the rewrite command, explaining how to modify existing snapshots.

It's a first draft focussing only on removing files.

[aawsome]

Thanks @s-leroux for the draft.

First, there are improvements for rewrite on the way giving more options and adding by default a rewrite tag. So, please wait until they land in nightly (maybe today or tomorrow).

One comment: instead of using rustic find to find affected snapshots, using -n outputs which snapshots would be modified by the given rewrite options (and omits the ones which are not affected) - so maybe this should be shown in the docu?