[Tooling]: Pin the Java toolchain and add contribution docs

[rugbedbugg]

[Tooling]: Pin Java toolchain

Adds mise.toml pinning java = "temurin-21.0.12+101.0.LTS".

The repository has had no toolchain pin, so every contributor's JDK came from whatever their global config or PATH happened to provide. CI already standardises on Temurin 21 (actions/setup-java@v4 in android.yml, desktop.yml and release.yml); this makes a local checkout match without relying on a developer's global mise config.

Verified locally — with the file in place, mise config ls reports the repository config alongside the global one and mise exec -- java -version resolves to Temurin 21.0.12.

[Docs]: Add community contribution guidance

Adds CONTRIBUTING.md (module boundaries, development setup, JVM-target and credential-hygiene rules, PR expectations) and CODE_OF_CONDUCT.md (Contributor Covenant 3.0, with private reporting through VinnovateIT maintainer channels).

SECURITY.md and README.md are already on main; these are the two remaining standard community health files GitHub looks for.

Notes

No build files are touched — Gradle does not read mise.toml, so this cannot affect CI behaviour or the resolved dependency graph.