chore(docs): add notes to default tags

[nitrocode]

what

• chore(docs): add notes to default tags

why

• Explain each environment variable
• Explain how to run local plans
• Explain colon usage (as per aws best tagging practices) in environment variables
• Use BASE instead of HEAD to avoid a forked repository from appending the tag instead of the base repository

tests

N/A

references

• https://www.runatlantis.io/docs/custom-workflows#add-directory-and-repo-context-for-aws-resources-using-default-tags