Split the howto into sections/files

[n-st]

We're accepting pretty much anything into the howto that might come in handy when setting up or experimenting with the SIP-DECT hardware or software — the less reinventing of wheels is required, the better.
That makes the howto page more useful, but also increasingly unwieldy.

We've long had the idea/plan to split the howto into thematic sections to make it a bit more manageable.

"Someone" would need to decide on useful splits, and massage Jekyll in such a way that as few existing links as possible break, and that we still get an (auto-generated) summary page for people to Ctrl+F in…

Additionally, we'll need a (separate) page about PPs at some point…