Document the release process

[SibtainOcn]

Writes down the release process, after 1.0.7 failed the F-Droid pipeline on two things that were both invisible until CI said so.

docs/RELEASING.md covers:

• The one command, tools/release.ps1, and the warning not to hand-edit the version alongside it.
• What has to be true before the tag: literal version fields, tag matching versionName, five store changelogs, the code moving by 100.
• The version code table, and the note that the same ABI offsets live in three files that must agree.
• The rules that keep builds reproducible: no dependency metadata block, no stripping of the bundled native library, no -PVERSION_NAME on the release path, no moving a published tag. Each one is there because it broke a release.
• How to verify the published bytes rather than the source change: the signing block IDs to expect and the certificate hash to compare.
• Updating the F-Droid recipe afterwards, including the trailing space fdroid rewritemeta requires and the single rebased commit the maintainers ask for.
• Where to look when a job fails, by job type.

Documentation only. No code changes.