get \index{...} working on PlanetMath

[holtzermann17]

Another point where Deyan and Bruce could usefully be involved is with
index terms. LaTeXML does support \index{...}, I think, but I'm not
sure how it would work in combination with PlanetMath articles.

E.g.:

• the indexed terms would get attached as thesaurus metadata items and resolve internal references to planetary nodes.
• the \index'ed terms would link to specific location on the web page (corresponding to the place the term was marked in the source).

[holtzermann17]

Note, we could use \index{...} interchangeably with pm:keywords, and doing it that way would be more portable.

[dginev]

We need to discuss locality. In LaTeX, \index accumulates metadata that creates the index for the single document you are processing.

Would you want to have the same functionality in PlanetMath? A site-wide index is a completely different feature that needs to be properly understood before we can start implementing.