Add `before:YEAR` and `after:YEAR` flags to `Search filters` docs

[cameronj86]

Works for YouTube and just realized it works for Invidious as well.

[cameronj86]

Actually my understanding is that most (all?) of these filters appear to work with Invidious? Might be worth a callout in the docs:

https://seosly.com/blog/youtube-search-operators/

[unixfox]

Feel free to submit a PR: https://github.com/iv-org/documentation/blob/master/docs%2Fsearch-filters.md

[cameronj86]

Sounds good. Are these built into Invidious or should I separate them out as related to the YT API or something similar?

[unixfox]

Feel free to add them after "Supported operators:"