docs: improve PR workflow

[kenjis]

Description

• add "If you sent to the wrong branch"
• add optional step to back up the PR branch

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• [] User guide updated
• [] Conforms to style guide

[datamweb]

💯

[MGatner]

Thanks for adding this! Seems a frequent need. One suggestion, take it or leave it.