Minor edits and updates for clarity and deduplication

[keithchev]

This PR makes a few minor changes:

• it revises the source control standards document to bring it up to date with current practices and reduce duplication with other documentation (for context, this document was originally written by Mert two years ago and had not been updated since).
• it adds more details about sensitive information to the standards for public repos.
• it renames the filenames of the guides and standards documents for clarity.

Note to reviewers: the changes to the source control standards document were substantial enough that it is rendered as a new file in the PR diff. Most of this document is unchanged or lightly edited, so please don't review too closely.

[taylorreiter]

Looks good to me!

[ekiefl]

At my last company we called this sort of work "noble", and made sure to give praise whenever it was done. So thanks for this noble work, @keithchev. It looks great.