* Remove stale entries from `.document` files
`doc/.document` referenced two non-existent directories (`date`, `rdoc`).
`ext/.document` referenced eight extensions that have been moved to
gems: bigdecimal, fiddle, monitor, nkf, racc, readline, syslog, and
win32ole.
These entries were silently ignored by RDoc. Verified that generated
HTML is identical before and after removal.
* Remove outdated and obsolete doc files
- `doc/yarvarch.en`: placeholder with just "maybe writing."
- `doc/yarvarch.ja`: 2005-era YARV design doc, predates Ruby 2.0
- `doc/forwardable.rd.ja`: old `.rd` format, not referenced anywhere
- `README.EXT`, `README.EXT.ja`: redirect stubs pointing to
`doc/extension.rdoc` since ~2015, with no live references
None of these files matched the `doc/.document` patterns so they were
never included in generated documentation.
* Remove redundant `charset` and `encoding` from `.rdoc_options`
RDoc defaults to UTF-8 encoding and its own source says "`encoding` is
preferred over `charset`". Both were set to UTF-8, matching the
default, so neither is needed.