docs: Add note to legacy notes

[mister-ben]

Description

People keep finding the v4 legacy docs at docs.videojs.com/docs and Google keeps positioning them highly in search results. This attempts to lessen that.

Specific Changes proposed

• Adds a note with a link to the top of each page.
• Adds a link with rel="canonical" to each page.
• The URL linked is the equivalent doc in current docs if my script could figure it out otherwise just docs.videojs.com

Putting a nofollow rule for /docs in a robots.text wouldn't hurt too.

[gkatsev]

Thanks @mister-ben! It's something I've wanted for a long time, thanks for actually getting to it.

[gkatsev]

If we want a robots.txt, we can just add it to the docs repo as a file