[META] Add CODEOWNERS files

- As per https://codeberg.org/forgejo/discussions/issues/53#issuecomment-1070207
- Using the `CODEOWNERS` feature it should speed up the development
process for contributors as for the most common PRs the right reviewers
will be added by Forgejo automatically. They can be added very
precisely according to the changed files of the PR.
- This feature is implemented in v1.21, which means it's not available
on Codeberg.

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CODEOWNERS

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+# This file describes the expected reviewers for a PR based on the changed
+# files. Unlike what the name of the file suggests they don't own the code, but
+# merely have a good understanding of that area of the codebase and therefore
+# are usually suited as a reviewer.
+
+
+# Please mind the alphabetic order of reviewers.
+
+# Files related to the CI of the Forgejo project.
+.forgejo/.* @dachary @earl-warren
+
+# Files related to frontend development.
+
+# Javascript and CSS code.
+web_src/.* @caesar @crystal @gusted
+
+# HTML templates used by the backend.
+templates/.* @caesar @crystal @gusted
+
+# Files related to Go development.
+
+# The modules usually don't require much knowledge about Forgejo and could
+# be reviewed by Go developers.
+modules/.* @dachary @earl-warren @gusted
+
+# Models has code related to SQL queries, general database knowledge and XORM.
+models/.* @dachary @earl-warren @gusted
+
+# The routers directory contains the most amount code that requires a good grasp
+# of how Forgejo comes together. It's tedious to write good integration testing
+# for code that lives in here.
+routers/.* @dachary @earl-warren @gusted