Added lemma to swap mathcomp natural log with Rocqs natural log

[lstrsrmn]

Motivation for this change

Adding a lemma to rewrite mathcomp's natural log with Rocqs constructive reals natural log

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

Merge policy

As a rule of thumb:

• 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.

Reminder to reviewers

• Read this Checklist
• Put a milestone if possible
• Check labels

[affeldt-aist]

Thanks for the contribution. Let me ping @t6s who recently added lemmas to this file that could be potentially related.

[proux01]

Rebased to fix changelog