docs(skills): Start adding code-review skills for Copilot

Add a few instructions how Copilot should review pull requests to reduce
the amount of repetitive comments done by reviewers.

Signed-off-by: Sebastian Schuberth <sebastian@doubleopen.io>

Author: sschuberth

.github/skills/code-review/SKILL.md

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+## Commits in the Git history
+
+As outlined in the [contributing guidelines], each commit in a pull request should adhere to the following rules:
+
+* The title should follow the [Conventional Commits] specification with the [commitlint configuration].
+* The description part of the title should start with an uppercase character.
+* The title should not exceed 75 characters in length.
+* Body lines should not exceed 75 characters and must be hard-wrapped with line-end characters.
+* The body should provide a rationale for the change and not just repeat the code diff.
+* Symbols should be enclosed in backticks so they render nicely in a Markdown view.
+* The language should be American English without spelling- or grammar-mistakes.
+
+## Tests written in Kotlin using the Kotest framework
+
+* Prefer the syntax where Kotest specs get the body passed as a lambda constructor argument.
+* Infix matcher syntax should be preferred.
+* Specific matchers like `collection should haveSize(2)` should be preferred over general matchers like `collection.size shouldBe 2`.
+
+[contributing guidelines]: https://github.com/oss-review-toolkit/.github/blob/main/CONTRIBUTING.md
+[Conventional Commits]: https://www.conventionalcommits.org/
+[commitlint configuration]: https://github.com/oss-review-toolkit/ort/blob/main/.commitlintrc.yml