Make single-page and PDF version more visible

[SonoSooS]

Currently the special versions of Pan Docs are hidden so much into the end of the Foreword section, that it's safe to assume that many people don't know these versions even exist, unless they know to exactly where to look.

Adding links of these to the bottom of the sidebar for example could boost visibility to these, as zero hunting would be necessary in that case.

[ISSOtm]

The printable version is already linked to in the top bar, so no need to do anything about it, IMO.

As for the "single page", here's an implementation:

• Have a src/single.md, containing some placeholder text (e.g. "don't edit, this gets overwritten"); link it preferably at the bottom of the sidebar
• In the renderer, overwrite the contents of single.html with print.html (tweaked, like we currently are)
• Done!

Once that's done, the links at the bottom of the foreword page can be deleted.

[coffeevalenbat]

Currently I use the pdf version of the pandocs (generated from the Print option at the top right of the page) as a local copy for when the page itself gets too slow (not sure if that is a me issue, but that's besides the point) and I noticed that most hyperlinks within the PDF version that link to other parts of the pandocs still link to the web version - So if there's a hyperlink for say, the rendering page, instead of forwarding to that page in the local PDF, it opens up the web link for it, which defeats the point. Not sure if this can be fixed or if it can't due to the way the page is built, but I figured it's worth mentioning.

[ISSOtm]

AFAICT, this is already fixed? I tested the links under this heading, and it redirected within the "single" page...

[ISSOtm]

Actually, this is tracked in #515!