Moved Jira to "out of use" as it's now been archived.

Also:

- Clarify that PRs don't need to have associated tickets
- Clean up some language here and there

GitLab doesn't allow spaces in the username.

I've changed the link for contribution guidelines to an internal link within the guide (rather than an external link to GitHub that links back to the guide).

Replace 'Wordpress' with 'WordPress' to use correct camel casing according to the WordPress Wordmark and Trademark

Reading for content; editing for consistency. 
Updated per seamuslee001's comment, and I found another missed edit in the automate testing paragraph.

Reading for content, updating for consistency (and minor changes for emphasis and clarity).

Reading for content; editing for consistency.

Reading for content; editing for consistency.

Reorder requirements docs into the basics section and move devloping civicrm into core and add backdrop in

This way MkDocs will be able to report errors for broken links

Done with:

perl -0777 -i -pe \
's@(?<!!)\[([^\]]*)\]\((?!https?://)(?!mailto:)(/(?:(?:(?:/(?![\)#]))|[^/)#])(?!.md))+)/?(#[^)]*)?\)@[$1]($2.md$3)@gm' \
`find . -name '*.md'`