[docs] Adjusted documentation to reflect that solids can also be tagged (issue 3291)

Summary: Added documentation for solid tags. Couldn't find how to access these tags in Dagit, but added examples of tags being used in pipeline execution.

Test Plan: Changed test to include solid tag in computation result

Reviewers: catherinewu

Reviewed By: catherinewu

Differential Revision: https://dagster.phacility.com/D6087