feat: better docs

[axe312ger]

We plan to finally get the docs of this repository into a good shape. Addressing plenty of complaints:

• Methods missing in the documentation #1304
• Documentation for JavaScript Plain API #1574
• How to filter entry.getMany? #1690
• Docs are unusable. How to create/manage releases? #1821
• Docs suck #1969

Goals

• clean up config and update typedocs
• (ongoing) improving current jsdocs comments
• ensure all types and interfaces are available, so people finally can fully dig around
• ensure already included examples are actually rendered
• clear introduction of both client types
• soft deprecation of legacy chainable client
• clear separation in navigation between plain and chainable client
• reintroduce structure for legacy client like in v4 and v5
• introduce a version selector
• potentially deprecate/discontinue outdated theme https://github.com/contentful/contentful-sdk-jsdoc/ and migrating https://github.com/contentful/contentful-sdk-jsdoc/blob/master/bin/publish-docs.sh directly into this repo, either as circleci action or github workflow

[axe312ger]

note to myself: remove this commented out files!