Revise article internal cross references

[StephDriver]

Background

Cross referencing, and returning back to the original position having followed up on a cross reference are important for the readability of articles. This needs to work better. Our current situation is outlined below:

Currently

• cross references to other sections (e.g. a 'see section 4') are not consistently marked up as internal links, even within the same article, some will be marked as links, some will be plain text.
• The indices used to indicate an endnote are not contextual links
• Material theme does allow for return from endnote to original position, but does not have contextual links for this, with all such links being 'carat' ^ which is also too small a target for accessible selection.
• for figures, there is no distinction made between the cross-reference link to the figure, and the link which enlarges the figure.
• references don't have a way back to the text, as described in #3242
• handling of references is also discussed in #3939

Task

• revise how cross references to other sections of the main text are handled
• improve endnote handling (contextual links)
• improve reference handling (contextual links, and ability to return to main text)