Issue #709: Rename `rules` to `metrics`

[nikosbosse]

Description

This PR closes #709. This PR closes #610

Following the discussion in #610 this PR replaces occurrences of "rules" with "metrics" in the code

(to be precise, I updated the code by searching and replacing

• rules_ with metrics_
• _rules with _metrics
• rules = with metrics =
• (rules with (metrics)

The PR also updates a few instances in the docs to make it clear that "rules" means "metrics and scoring rules"

Checklist

• [x] My PR is based on a package issue and I have explicitly linked it.
• [x] I have included the target issue or issues in the PR title as follows: issue-number: PR title
• [x] I have tested my changes locally.
• [x] I have added or updated unit tests where necessary.
• [x] I have updated the documentation if required.
• [x] I have built the package locally and run rebuilt docs using roxygen2.
• [x] My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
• [x] I have added a news item linked to this PR.
• [x] I have reviewed CI checks for this PR and addressed them as far as I am able.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.28%. Comparing base (5db9070) to head (cab02d6).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #710 +/- ## ======================================= Coverage 95.28% 95.28% ======================================= Files 21 21 Lines 1632 1632 ======================================= Hits 1555 1555 Misses 77 77 ```

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