Leftover tasks for shared channel types

[sappelhoff]

following:

• #919

• [ ] rendering the channel type tables in the modality sections from the schema

• [ ] creating a new appendix page rendering ALL channel types

  • [ ] put links to this appendix below the modality ch type tables
  • [ ] explicitly mention that all channel types are shared within BIDS

• [x] adding remaining enums in the schema (or adding comments saying that we won't add them as of now ... for cases where such comments are not currently there already): https://github.com/bids-standard/bids-specification/pull/1640

Originally posted by @sappelhoff in https://github.com/bids-standard/bids-specification/pull/919#pullrequestreview-1494608953

[sappelhoff]

taking off the 1.9 milestone, as this is mostly visuals that can potentially be added after a release.