Update CMORizer documentation with command options

[remi-kazeroni]

Description

In various PRs and discussions I had with other developers, it turned out that a few things are missing from the CMORizer documentation:

• Mention esmvaltool data --help to get help on CMORizer commands
• Usage of --start and --end for downloaders
• Usage of --start and --end for formatters

• Closes #issue_number
• Link to documentation:
  • https://esmvaltool--2795.org.readthedocs.build/en/2795/input.html#observations
  • https://esmvaltool--2795.org.readthedocs.build/en/2795/develop/dataset.html

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[axel-lauer]

Thanks for updating the documentation! The changes look fine to me.

[valeriupredoi]

very welcome additions, many thanks @remi-kazeroni 🍺