Update the link/description of the browser extensions

[nicoonoclaste]

The website links to browser extensions (for Chrome and Firefox) now point at the “Silk” PrivacyPass client, which itself links to a repository under the @cloudflare organisation.

I believe it would be helpful to clarify on the website itself that the linked extension is not challenge-bypass-extension, and that the latter does not implement the protocol standardized as RFC 9576/9577 by the IETF.