Fix back_to_top not displaying when no other footer variables are set

[mattxwang]

This is the minimum necessary change to make back_to_top work when there is no custom footer, last edit timestamp, or GitHub edit link.

Two immediate thoughts. First, we have a pair of variables: back_to_top and back_to_top_text. In my opinion, this seems a bit unnecessary; we could just use a back_to_top, and treat any non-nil/false value as the text. We could make this backwards compatible (i.e. support but deprecate back_to_top_text). Any thoughts here?

Secondly, some of these conditions are weak:

just-the-docs/_includes/components/footer.html

Lines 7 to 9 in a251382

	{% if site.back_to_top %}
	<p><a href="#top" id="back-to-top">{{ site.back_to_top_text }}</a></p>
	{% endif %}

Here, this conditional should also check for back_to_top_text, and presumably, this should "bubble up" to the overall if statement on line 4 (similar things for the gh_* variables - the line 4 condition only checks for gh_edit_link). Is this a reasonable concern/take? If so, I can approach this some time (either in this PR, in another PR, etc.).

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After we decide the correct behaviour of back_to_top, I'll add documentation to the "Configuration" section!

Separately, @pdmosses mentioned:

The obvious fix is to always render the footer when site.back_to_top is set. However, it would improve the usability of the back-to-top feature if individual pages could override the site setting (to suppress the link on some short pages, or to show it on some long pages).

Happy to do that too - for organizational purposes, I'll punt that to another PR (that I can merge into the same release).

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Fixes #1443.