Add "documentation:" field to scheduled pipelines & processors

Context

Closes #837 (closed)

What does this MR do?

Scheduled pipelines

• We add an optional documentation: key to the Variant objects (defaults to '')
  • Rationale for the Variant object: Every scheduled pipeline has at least one variant
  • description: is already a reserved keyword, as it's the name we use to set the description of a scheduled pipeline (which is also used as an identifier).
  • The descriptions have not been written (I think it would be faster if we splitted those in the team)

Processors

• A documentation method has been added to all "child processors" (see below)
• Only "leaf processors" will need to have documentation (e.g. we don't need to document AppSecProcessor base class)
• The descriptions have not been written (I think it would be faster if we splitted those in the team)

Expected impact & dry-runs

None

How to test locally

# Checkout the branch locally
git fetch && git switch 837-add_description_field

# Create a new branch
git checkout -b remove_me_testing_branch

# Remove the last commit
git reset --hard HEAD~1

# Run the specs: 43 specs should fail
bundle exec  rspec --format=doc -e 'contains documentation'

# Go back to the main branch
git checkout 837-add_description_field

# Run the specs: 43 specs should pass
bundle exec  rspec --format=doc -e 'contains documentation'

# Clean up the branch we created
git branch -d remove_me_testing_branch

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Engineering week-in-review