Fix canonical tag

[dgarcia360]

Closes #405

Adds dynamically the correct canonical URL:

• If multiversion is set, the canonical URL has the form: <link rel="canonical" href="{{ html_baseurl }}/{{latest_slug}}/{{ pagename }}.html"/>

• Otherwise: <link rel="canonical" href="{{ html_baseurl }}/{{ pagename }}.html"/>

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[annastuchlik]

I don't think we can check it with the preview, but the changes look good to me.

[tzach]

@dgarcia360 not sure I understand the change
Can you please give a few examples, for latest and not latest versions?

[dgarcia360]

@tzach

• For the page https://sphinx-theme.scylladb.com/branch-1.0/getting-started/index.html (non latest version), the canonical url would be: https://sphinx-theme.scylladb.com/latest/getting-started/index.html

• For the page https://sphinx-theme.scylladb.com/latest/getting-started/index.html (latest version), the canonical url would be: https://sphinx-theme.scylladb.com/latest/getting-started/index.html

• For the page https://docs.scylladb.com/getting-started/install-scylla/index.html (without multiversion), the canonical url would be https://docs.scylladb.com/getting-started/install-scylla/index.html

[tzach]

LGTM