docs: avoid footnotes consisting of just URLs

docs: avoid footnotes consisting of just URLs

Replace the footnotes with inline links whenever the footnote text
consists of nothing but the URL.  While at it, make the link texts
consistent in the surrounding areas, for example avoiding usage of
"here" for the link's text.

In the case of acpi-bits.rst this fixes a build failure with Sphinx
8.1.0, because the FOSDEM link was duplicated in the paragraph and the
new version is a lot stricter about unreferenced footnotes.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>