New topology lemmas

[amolinamounier]

Motivation for this change

Split from #2023.
Generalized and moved lemma cvg_at_right_left_dnbhs from metric_structure.v to num_topology.v

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

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• PRs with several commits that make sense individually and that
  all compile are preferentially merged into master.
• PRs with disorganized commits are very likely to be squash-rebased.

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