Add rel=canonical tags to /docs

[fabicastp]

This is part of our work to improve SEO.

All of the Docs versions are missing rel=canonical tags. Adding these can be helpful for ensuring that users searching Google for Docs topics find the right version.

TO DO: Add canonical tags to our new docs -- soucregraph.com/docs

[gitstart-app[bot]]

Here is the GitStart Ticket for this issue: https://clients.gitstart.com/sourcegraph/3436/tickets/SGM-6775

[fabicastp]

Issue back in backlog until further notice (probably next week).

FYI, the new docs are public on github.com/sourcegraph/docs

[CommanderStorm]

@fabicastp Small headsup (just came across this issue by accident): This issue is likely resolved as https://github.com/sourcegraph/docs/pull/214 is in main ^^