Add canonical links for Google

[gilgongo]

==> UPDATE <===

These changes are probably better incorporated with @ignotus666 changes to the language selector. So putting in draft for now.

Short description of changes

1. Fix for [Bug] Untranslated pages make Google complain about duplicate content #986
2. Adds fully-qualified links to KB articles in sitemap.xml which I think might be responsible for "invalid URL" errors from Google.

What is missing until this pull request can be merged?
Ideally, need to work out how to get _includes/general/nav.html to work, but it's not doing it when the current language is en for some reason.

BTW I'm not 100% sure that adding rel=canonical to the EN version of links is necessarily going to fix Google's problem, since they imply we can only "suggest" the canonical versions, but hopefully this will make things a bit better.

Does this need translation?

No

Checklist

• I've verified that this Pull Request follows the general code principles
• I waited some time after this Pull Request was opened and all GitHub checks completed without errors.
• I'm sure that this Pull Request goes to the correct branch

[gilgongo]

BTW the only way I can get the site to work without errors after cloning it locally (on Ubuntu 24.04) is to:

bundle add webrick
then change to liquid (4.0.4) in Gemfile.lock
(Also had to use po4a v0.69 but @ignotus666 gave me v0.72 which also works)

[ignotus666]

I had a go yesterday and surprisingly enough didn't run into any problems - the usual commands worked. I'm on Mint 21.3 (based on Ubuntu 22.04).

[gilgongo]

Ok so maybe it's some ruby version dependency thing. I'll see if I can reproduce it on a fresh 24.04 installation and maybe add a note to the docs since the error messages are pretty unhelpful.

…

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[gilgongo]

I think we may be best merging this after the language code fix is merged, since Google might treat rel=alternate differently if it also knows the URL is in a given language.