Add more links to spaCy docs

[svlandeg]

Description

Add more direct links to the various built-in tasks that we provide.

I would perhaps also consider rewriting the external links for the models, to specific links in the API docs on spaCy (where you can often find more information & external points for the models, as well)

Corresponding documentation PR

NA

Types of change

readme update

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran all tests in tests and usage_examples/tests, and all new and existing tests passed. This includes
  • all external tests (i. e. pytest ran with --external)
  • all tests requiring a GPU (i. e. pytest ran with --gpu)
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[rmitsch]

Thanks!