Feat(cicd_bot): Document and enable the min_intervals plan option

[erindru]

Closes #4069

This PR:

• Adds documentation for the --min-intervals option which can also be used from the CLI
• Exposes this option in the CI/CD bot as pr_min_intervals (to be used with default_pr_start)

Note that I called it pr_min_intervals to align with the other pr_* options (pr_environment_name and pr_include_unmodified)

This achieves #4069 by specifying the following config:

cicd_bot:
  default_pr_start: '1 week ago'
  pr_min_intervals: 1

I decided not to introduce the suggested default_pr_relative_start parameter because default_pr_start already supported relative dates.

Instead, I made the "1" in this configurable (as the pr_min_intervals option):

If the computed start/end range does not cover a complete interval, then move start further back until 1 interval is loaded

The logic to give every model its own start date and ensure it covers --min-intervals intervals was added in PR #4780