CIAO: add [--list-pipelines]

What

Adds the option --list-pipelines to CIAO.

Why

1. To document existing pipelines, when they are triggered and what they do.
2. To make this documentation inlined, such that it lives close to the source code.

How

By adding a description field to pipelines (including child pipelines), and filling it for all existing pipelines.

Manually testing the MR

$ source scripts/version.sh; dune exec ci/bin/main.exe -- --list-pipelines
# or
$ make -s -C ci list-pipelines
# or, if you have docker but no OCaml compiler installed:
$ make -s -C ci docker-list-pipelines

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