Installation only linked out to the download page, and Features restated the
site's feature cards without saying what any of them do. The README now carries
the requirements, the per-platform install steps, and a Configuration section -
including that most features are off by default and that applying a change needs
a restart, which nothing in the repository stated anywhere.

The Japanese translation is dropped rather than kept in sync; lc.m1sk9.dev
serves Japanese documentation already.

Co-Authored-By: Claude <noreply@anthropic.com>

Paper 26.2 bundles Adventure 5.2.0, a major bump from the 4.26.1 shipped by 26.1.
Adventure 5 makes ClickEvent generic, so the raw type in
CommandContext.replyWithEvent no longer compiled. Auditing the rest of the
Adventure 5 removals found no other affected usage.

api-version is raised to 26.2, dropping 26.1 support: the two Adventure majors
are not binary compatible, so claiming 26.1 compatibility would be a lie.

Also fixes two problems found while verifying on a real server: the Velocity
healthcheck invoked mc-health, which does not exist in the mc-proxy image, so the
container had never once passed its check; and the debug environments pinned no
build at all, which meant the proxy came up as 4.1.0-SNAPSHOT while the plugin is
compiled against velocity-api 3.5.1. Server versions are now derived by x from
the Gradle coordinates so they cannot drift from what the plugin targets.

- Add README.ja.md (Japanese translation) with language picker
- Add language picker to README.md
- Add Velocity integration overview focusing on protocol compatibility
- Unify download link to lc.m1sk9.dev/download
- Fix broken license badge with a static GPL-3.0 badge

Co-Authored-By: Claude <noreply@anthropic.com>