x/pkgsite: hide deprecated things from documentation page by default

[julieqiu]

Related to #17056, we should hide deprecated things from the pkg.go.dev doc page by default.

[nasirhm]

I would like to work on it.

cc: @julieqiu

[julieqiu]

Thanks @nasirhm! If you're interested in working on this feature, please share a plan on this issue first. It looks like @dsnet might have also worked on a prototype in the past based on #17056 (comment).

[myitcv]

@julieqiu @dsnet - is there any update on this?

[myitcv]

Relaying a message from 0xjnml on Slack:

That's IMO a very bad idea. Please imagine the confusion when someone wants to check the docs for old code using something deprecated. It makes the UX frustrating, non-deterministic and unreliable. The docs shown on the web page should be the same as what is in the sources, only slightly modified by applying styles as usual. An added list of deprecated things is okay

[seebs]

That's a good point, it seems really bad to have documentation for a thing just magically go missing. Clear deprecation warnings would be good.