docs: move inline documentation from README to project website

Replace ~870 lines of detailed setup, configuration, and API usage
documentation in README.md with a slim overview linking to
a2aproject.github.io/a2a-java. Migrate the removed content into
docs/content/server.md (configuration system, task authorization,
user identity, v0.3 backward compatibility) and docs/content/client.md
(push notification config, REST transport, v0.3 client notes).

Signed-off-by: Emmanuel Hugonnet <ehugonne@redhat.com>

Author: ehsavoie