CIAO: add [~description] to jobs, can browse with [--describe-pipeline]

What

• Add a field for descriptions of jobs in CIAO.
• We also add a command --describe-pipeline for printing the list of jobs in a given pipeline with their description.
• And a command --overview-pipelines that prints the set of pipelines as a table.

Why

In !14881 (merged) !14275 (merged), we added descriptions to pipelines, and a --list-pipelines for browsing those descriptions. We now extend the inline documentation to jobs.

How

• Added code mostly self-evident from the description.
• Implementation of markdown tables for the output of the added commands.

Manually testing the MR

$ dune exec ci/bin/main.exe -- --overview-pipelines
# use a real pipeline name e.g. 'master_branch' and a dummy one e.g. 'plop'
$ dune exec ci/bin/main.exe -- --describe-pipeline NAME 

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR