[Lens] Improve api docs

[flash1293]

Right now the Lens API docs are not very comfortable to use and should be cleaned up:

• Do not export internal things (e.g. TopValuesResult or LensServerPlugin)
• Add explanatory documentation to all exported types
• For each of the internal services (datasource, various visualization types), create a public_types file and move all exported types there. Re-export from the top level

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[wylieconlon]

The goal of this is to provide good documentation of the Lens public API for use by solutions and end users, so I assume that we will also want to expose a few more types publicly like IndexPatternLayer- as this is not currently exposed?

The main use cases I am imagining for users who will read the API docs:

• Solution teams who are embedding Lens
• Administrators who want to automate dashboards and need to understand the saved object format

[flash1293]

@wylieconlon All of these types should be exposed - see here

kibana/x-pack/plugins/lens/public/index.ts

Line 10 in a2c47ef

export {

IndexPatternLayer specifically isn't exposed, but only because it's not part of the embeddable input - it's PersistedIndexPatternLayer.

Totally agree about the use cases you listed - these are the people we are targeting.

[stratoula]

Closing in favor of #152553