doc: use consistent capitalization for addons

Sometimes _addon_ is capitalized and sometimes not. Capitalizing it
seems peculiar and hard to justify. Standardize on treating it like
other common nouns.

PR-URL: https://github.com/nodejs/node/pull/34536


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>