Navigation aide

From a recent discussion, I think the biggest improvement that could be made to the gf-guide content would be to make it easier to get around.

The biggest impact would be a search feature (which is already open at #95); without it users have to go to an external site to find a specific subject.

The next best thing, though, would be to have a sidebar or some other on-page navigation aide, so readers are able to jump to a specific other page in one step. The current pages each have two or three links to other pages in header and/or footer positions, but which pages those are is somewhat arbitrary -- and some pages have both header and footer lists, but others have one but not the other. If you can't find a link in the text itself to something you want to cross-reference, all you can do is back up to the index page and look for it in the TOC, which isn't ideal.

For instance, the [diacritics](https://googlefonts.github.io/gf-guide/diacritics.html) page says that the designer needs to be aware of specific language requirements, including tag names, but it doesn't link to the [lang](https://googlefonts.github.io/gf-guide/lang) page. It does link to the overall-requirements page to point them to the mention of [glyphsets](https://googlefonts.github.io/gf-guide/requirements.html#glyphsets) (although that link's broken at the moment) so it's hard to find where the lang explanation lives. So, it's a closely related concept, but hard to get to.

A full index listing in a sidebar or header/footer would solve that, as well as giving readers a way to mentally orient themselves in the overall doc, which is also good.