Add table of contents to the specification

[matklad]

Hi!

Currently, the spec is a bit hard to read because there is no ToC which lists all methods/notifications and types. It might be a good idea to provide one! Take a look at the index at the bottom of the Dart server spec, for example, it is really convenient: https://htmlpreview.github.io/?https://github.com/dart-lang/sdk/blob/master/pkg/analysis_server/doc/api.html#Index.

[vladdu]

Also it would help to make it more visible which messages are client->server and which ones are server->client.

[vladdu]

I have added a TOC manually, but it would have been nicer if there was a machine-readable schema that could be used to generate the docs. That would allow to easily cross-reference even the type descriptions, for example. It would help for #99 too, by allowing stubs to be generated.

[dbaeumer]

Agree and I have an item to spec the protocol in JSON schema. Then we can have tools for various outputs. What do you think about using JSON schema for this.