Restructure documentation

[jorainer]

• Organize Spectra documentation into thematically connected documentations.
• Add aliases for all methods to avoid e.g. ?intensity ending up in the documentation in ProtGenerics.

[jorainer]

This fixes #288

[jorainer]

@philouail , no need to go through the full documentation - maybe just some feedback if you're OK with the structure now or if it would make sense to further divide the documentation into smaller parts.

[philouail]

Not the easiest to read in Github I think aha. But this looks great, i'm super happy, thanks !