docs: mark project rules in the rules list

[bhirsz]

Makes project rules visible in the documentation and in the CLI:

• new Rule.project_rule attribute,
• generated rules list renders a Project rule note with a link to the project checks docs,
• robocop list rules tags such rules with [project],
• new robocop list rules --filter PROJECT,
• the repeated 'This rule is a project level rule' paragraph was removed from the rule docstrings,
• test asserting that project_rule matches rules reported by ProjectChecker.

Stacked on #1828.