Tweaking the default HTML template

[tajmone]

@pml-lang, I've added to the repository the HTML_TEMPLATE.md document as an attempt to document the default PMLC template, but also added some notes and considerations on how it might be improved to aid the creation of custom stylesheets.

A this stage, any changes to the hard-coded PMLC template shouldn't really qualify as "breaking changes" since there's no official definition of the template, its classes, etc. So, hopefully, these discussions might lead to tweaks and improvements, from one PML release to the other, which might move forward the creation of better native stylesheets, and also define some standard criteria of how PML injects IDs and classes on the various elements.

As I've written in the HTML_TEMPLATE.md doc, the main problem with the current template is that it only injects classes, even on unique layout elements (doc-wrapper, header, footer, sidebars) which would greatly benefit from an ID.

I also wanted to ask you if there's a reason for each and every class having the pml- suffix. Wouldn't it be better to adopt prefixes which carry semantics, e.g. like in the naming rules proposed by SMACSS where l- indicates Layout Rules, is- indicates State Rules, etc.?

Although there might be some elements where it does make sense to preserve the pml- prefix, e.g. for PML elements which have no HTML counterpart (admonitions, footnotes, etc.), it might be overkill to have every single class qualified as pml-.

The ToC HTML tags and classes definitely need tweaking, but that might deserve a discussion thread of its own, whereas here I'd like to focus on more general considerations about the state and future of the PML template.