doc: the typo train

Context

This is the current edition of the "typo train", a periodic MR for fixing typos and minor errors in the documentation. Feel free to contribute with commits to this branch.

The minor changes in this edition beyond pure typos are:

• add a brief section for Style guidelines in docs/README.rst (Issue #919 (closed))
• add a section about writing Executable documentation in docs/README.rst
• explain why dir is mutable while rev_path is not (Issue #1109 (closed))
• fix unfinished sentence (Issur #924 (closed))
• revive Tezos Baking slack
• link for creating issues in page 404
• fix duplicated History pages in the left menu for User documentation
• explain the typo train in contributing.rst

Manually testing the MR

Reviewers must check whether commits respect the above criteria and improve the documentation or are otherwise useful.

Checklist

N/A.