docs: clarify Android license and contribution workflow

The README calls Android public domain even though the repository ships GPL v3, and the PR template links to a missing contribution section. Point to the existing license, add a contribution guide based on the repository's current build and testing requirements, and make the README/template links resolve.

Fixes #941. Fixes #654. Fixes #162.

Validation: all local Markdown links in the new guide and README resolve; git diff --check passes. Documentation only; no application behavior or license file changed. The separate website report in #727 is not covered by this repository change.

• Read the repository contribution guidance
• Reviewed the diff
• Linked the issues addressed
• Reported validation; application tests are not applicable to this documentation change