doc: Make HTML ids discoverable

doc: Make HTML ids discoverable

In the HTML output, this decorates section headers and variable list
terms with a marker ("#") that is a link to the same section/term.
That way, links inside a page can be discovered for easier sharing.
The marker only appears when hovering.

This now requires that all elements that are candidates for such a
link have an id attribute.  Otherwise, an error will be generated.
All previously missing ids have been added prior to this patch.

Author: Brar Piening <brar@gmx.de>
Reviewed-by: Karl O. Pinc <kop@karlpinc.com>
Discussion: https://www.postgresql.org/message-id/flat/CAB8KJ=jpuQU9QJe4+RgWENrK5g9jhoysMw2nvTN_esoOU0=a_w@mail.gmail.com