doc: improve user/snapshots.rst

Context

This MR proposes some improvements to the existing page https://tezos.gitlab.io/user/snapshots.html, including low-level fixes (typos, syntax, more coherent notations in the code excerpts, etc.), and a few more semantic changes (adding a small intro, removing redundant paragraphs, etc).

Manually testing the MR

Check that the changes improve readability and are accurate.

Checklist

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