Cleaned up formatting in release-guide.md according to our guidelines.
Cleaned up formatting in release-guide.md according to our guidelines.

This change includes:
* Automatic enumeration of lists.
* Title case capitalization.
* Correct spacing between sections.
* Correct indentation of text and code snippets.
* Correct decoration of notes and code snippets.
* Wrapped text for readability and diff-ability.

Review: https://reviews.apache.org/r/53384

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