Add mdBook-rendered manual

[ISSOtm]

Currently rendered at https://eldred.fr/hUGETracker, re-rendered on every modification made to manual/** on the hUGETracker branch.

This is only a fairly direct port of the original manual, which is known to be outdated. (I know the effects section is, for example.)

Things to do:

• Proofread this (I'm not 100% sure of the language I used everywhere)
• Take new screenshots where relevant (Windows screenshots should be taken, for consistency with what's already there)
• Update information where necessary
• Should a table be used for the effects reference? I'm thinking that one ## 0xy — Arpeggio heading per effect may work better instead, including on the editing side
• Decide under what URL this should be hosted; as-is, this would be superdisk.github.io/hUGETracker, but you may want to use nickfa.ro instead? (This can be done later, though.)
• Once Overhaul audio section gbdev/pandocs#350 is merged, replace the links pointing to eldred.fr/pandocs with gbdev.io/pandocs.

[ISSOtm]

Regarding the infomation updates, my main point of contention lies on the effects reference.

Here are some claims that we may want to update, though imo based on what we want the behaviour to be, not what the behaviour is. (I'm willing to make the necessary modifications to both hUGEDriver and fortISSimO.)

• Can toneporta be used in a cell with an instrument?
• How should the vibrato speed argument be handled?
• How should routines be called? Should some of the bits be reserved for arguments, and if yes, how many?
• Should the current hUGEDriver behaviour for 9xx (nominally "set duty cycle", but since then extended for CH3 and CH4) be considered canonical?
• Is volume slide still meant to trigger once per tick, or only on the first one?
• Is volume slide meant to work on a cell with a note or instrument?
• What is the exact behaviour of volume slide, clamping-wise? Or should at least one of x and y be zero?
• Bxx on the same row as Dy should jump to row yy in order xx, right?
• Is Cxx meant to work on a cell with a note or instrument?

Additionally, the routines' calling convention should be documented.