Avoid mixed content in Respec's documentation.

[jyasskin]

Most of these are just the examples' <script src> tags, but src/conf/logos.html
also included an example of sourcing an image over HTTP.

You can see this broken now if you click the link from https://www.w3.org/respec/guide.html#contiguous-idl.

[marcoscaceres]

Awesome. Hehe, guess people are not accessing the docs too much :)

[darobin]

Actually I suspect most people are still accessing it as http since that's what the original link was. I'd force-upgrade for that directory but I don't want to be drawn into the update-all-w3.org-to-https project :)

[jyasskin]

Yeah, I think I landed on it as https because Google started returning that result for [respec], but if I'd bookmarked it, I'd probably still be on http.