* docs: simplify plugins documentation
* docs: restore plugin caveats lost during simplification
Re-add three behaviors that were dropped from the simplified plugins
docs: the stdio MCP `cwd` must start with `./`, local-path installs run
from the managed copy (so editing the source after install requires a
reinstall), and `/plugins remove` only deletes the install record while
leaving files on disk. Mirror the changes in both en and zh.