Create documentation for new Channels API

[rikoe]

Documentation needs to be created on the FDC3 Docusaurus website for the new Channels API changes merged via #94.

This involves:

• Creating a new Channel.md file under https://github.com/FDC3/FDC3/tree/master/docs/api, using https://github.com/FDC3/FDC3/blob/master/docs/api/DesktopAgent.md as an example
• Ensuring that all the methods and properties from https://github.com/FDC3/FDC3/blob/master/src/api/channel/channel.ts are properly documented in the markdown file in easily understandable language, with included usage samples and with level 3 markdown headings, so that they show up as clickable links in the right-hand navigation section
• Editing https://github.com/FDC3/FDC3/blob/master/website/sidebars.json to include the new markdown document
• Ensuring the API interface documentation is consistent with the specification at https://fdc3.finos.org/docs/next/api/api-spec#context-channels

Once merged, the website will be updated for the master branch, visible at https://fdc3.finos.org/docs/next/api/api-intro

Docusaurus authoring help: https://github.com/FDC3/FDC3/blob/master/website/README.md#adding-a-new-docs-page-to-an-existing-sidebar

[rikoe]

Addressed by #154 and #161.