Docs: Glossary / overview of concepts

We need a glossary section in the documentation site that defines and explains mooltipage-specific concepts and terminology such as "fragment" and "expression". This should be in the "reference" section of the documentation site (reference section does not exist yet, but it can be created for this).

An example of the kind of information that should be included:

• Fragment: A fragment is an isolated DOM section containing "HTML-ish" content. A fragment can be imported into another fragment or page which makes it into a "component". Fragments that can be compiled into valid, standalone HTML are referred to as a "page".
• HTML-ish: HTML content where syntax is enforced, but most semantics and constraints are removed. HTML-ish context enforces only a small subset of actual HTML restrictions and special behavior, such as parsing , , and elements as containing only text. Other restrictions, such as position and element count, are not enforced.
• Component: // TODO
• Page: // TODO

For this issue, then glossary does not need to be comprehensive or complete. It just needs to be created and have enough information to show how it should work.