Document removal of LFS objects from repo to reduce size

What does this MR do?

Based on a customer report (internal), I've been testing our guide with respect to LFS files a bit. I'm not quite certain yet, whether a step is definitely missing, or if this addition is the answer.

In any case, it seems like some step to specifically make LFS items "unused" is missing, so that the final cleanup can actually "unlink" them.

@eread: In my initial tests, the commits that reference an LFS object needed to be removed/pruned from the history. git filter-repo does not do this, does it? I'm looking at newren/git-filter-repo#257 & newren/git-filter-repo#355 and think not, because the small pointer files will almost never be highlighted as items to be removed, when users follow the gist examples in our guide. Thus, I'm thinking that we should add 1 or 2 sentences about how to approach removing LFS objects also from the server specifically. The prune part of this MR is only for local LFS objects.

Related issues

• companion to Reducing repo size docs: Clarify "commit-map" s... (!92338 - merged)

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