use new HtmlId and HtmlIds classes

ids are represented by simple strings, and are created inconsistently throughout the standard doclet, including in SectionName, Links, and with miscellaneous string constants.

It would be good to introduce a type-safe wrapper class, HtmlId, and then centralize the management of ids in a new factory class, HtmlIds.

As a followup up, it would then be good to review the names of the ids we generate, to attempt a more consistent naming scheme.