Add an option to generate a table of contents for large API references

[Chriscbr]

This was available in jsii-docgen v1 but I think the enhancement was removed in v2. On https://constructs.dev/ we provide a table of contents automatically, but adding a table of contents within the generated markdown would make it easier to see all of the classes, structs, enums etc. that are available at a glance when rendering the docs on GitHub, GitLab, etc.

Example: https://github.com/projen/projen/blob/v0.46.0/API.md