Update/branch naming

[jeffpaul]

Description of the Change

This PR begins the branch renaming efforts to trunk (and in the case of GitHub Pages-powered sites to gh-pages) from master as noted by @pcrumm in #62.

Note that for this PR to be fully resolved the following need to be updated, so please either make those link changes in this PR and merge ...OR... wait to merge this PR until those changes are made in separate repos and then update those links in this PR and merg:

• https://10up.github.io/Engineering-Best-Practices/version-control/#protecting-the-master-branch; see --> change master to trunk in Version-Control.md Engineering-Best-Practices#337
• https://github.com/10up/actions-wordpress/blob/master/hookdocs-workflow.md
• https://github.com/10up/.github/tree/master/

Alternate Designs

n/a

Benefits

Aligns to 10up's "inclusivity in code".

Possible Drawbacks

None identified besides negative troll comments.

Verification Process

Manually verified via GitHub Desktop UI.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

Relates to #62.

Changelog Entry

## Changed
- Branch naming from `master` to `trunk` (or `gh-pages` as necessary)