Fix typos and punctuation in the 'docs.md'

* Add commas to introductory phrases ('However', 'First', 'For example')
* Consistently capitalize provider names
* Fix some typos ('bandwith', 'integriTIty', etc.)
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f