CLI Adjustments and Documentation

[joshmeek]

Thanks for contributing to Prefect!

Please describe your work and make sure your PR:

• adds new tests (if appropriate)
• updates CHANGELOG.md (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

Note that your PR will not be reviewed unless all three boxes are checked.

What does this PR change?

Closes #1266
Closes #1267
Closes #1268

Removes the auth add command in favor of auth login. Removes the empty summarize group. run --logs now auto exits on flow completion. Adds general documentation for CLI.

Why is this PR important?

Removing unused CLI cruft and adding CLI documentation are great steps for user clarity

[codecov]

Codecov Report

Merging #1319 into master will decrease coverage by 0.1%.
The diff coverage is 62.5%.

[cicdw]

Some minor typo fixes, but otherwise this is awesome! I think this sheds a lot of light on Cloud's API that people will find interesting.