Add an idiom for conditional flow control

[jcrist]

This adds an idiom for conditional flow control to the docs. This covers both case and merge, which (IMO) are the main flow control constructs a Prefect user should need. Note that since this (and a few other idioms) overlapped with the branching idiom, I removed that one in favor of this.

This also adds an optional flow kwarg to merge to make it easier to use with the imperative api.

Fixes #2530.

• adds new tests (if appropriate)
• add a changelog entry in the changes/ directory (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

[jcrist]

The test failure appears to be unrelated.

[codecov]

Codecov Report

Merging #2727 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

[joshmeek]

👍