Add table of content

[dblanchette]

For big schemas, it would be nice to have a table of content.

It would basically be a lightweight version of the schema without the description and definitions.

Each element would be a link to the actual definition in the schema below.

This feature needs to be optional, activated with a config option.

[fchastanet]

I implemented a toc in md schema (see PR https://github.com/coveooss/json-schema-for-humans/pull/76)