RFC: unify pipeline states & colors and document them

[anbraten]

We should unify all names, colors and meanings of pipeline states throughout Woodpecker and finally document the states with a description to our docs.

Currently the state is displayed to the user at following places:

• badge (text with color)
• favicon (colored dot)
• UI pipelines lists (colored icon with colored bar) and
• UI pipeline log (colored dot & colored icon at top)

The following states do currently exist in the source-code:

• skipped: Pipeline got skipped because its dependencies haven't ended successfully or not at all (exp canceled)
• pending: Pipeline is waiting for an agent to pick it up
• running: Pipeline currently being executed by an agent
• success: Pipeline ended with exit code 0
• failure: Pipeline ended with exit code < 0
• killed: Pipeline got killed (exp canceled)
• error: Pipeline could not be started (exp container image not found)
• blocked: Pipeline is waiting for the confirmation of an repo admin
• declined: Pipeline got declined by an repo admin