Appendix A.1 removed, LINK section revamped

[lmichel]

This PR is inspired by the proposal made by Pierre Fernique's about LINK at last interop (06/26).

• Appendix A.1 has been removed as suggest in Review Appendix A, "Possible VOTable extensions" #53.
• The use of templated LINK@ref has been described in a more generic way (context free).
• The use of templated LINK@ref has been moved to the LINK section.
• The processing of templated URLs is now described a more generic way in that normative section.
• The LINK section has been revamped for clarify
• change section updated accordingly

[mbtaylor]

@lmichel thanks for this PR which moves LINK templating into the body of the text; that might be the way to go though I think it still needs a bit more discussion.

However I note it also introduces into the main text two new values for the content-role attribute, "query" and "hints".

content-type="query" refers to the "VOTable Query Extension" subsection of Appendix A (A.2 in VOTable 1.5; A.1 in this PR). I struggle to understand the content of that appendix, as far as I know(?) it's not used anywhere, and it relies on some old/outdated standards (Astrores and ASU) - it's one of the appendices I was hoping to get rid of. So I'm not much in favour of including this option in the text.

content-role="hints": is this currently used anywhere? The suggested description: "indicates a pointer to information to be used by use by the application." doesn't say anything concrete about what you'd do with it.