Leftover tasks for shared channel types

[sappelhoff]

following:

• [SCHEMA] Add full object definitions for valid values in schema #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

• 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): [SCHEMA] Add remaining enum definitions #1640

Originally posted by @sappelhoff in #919 (review)

[sappelhoff]

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