Add an index page for the docs

[pleshakov]

Is your feature request related to a problem? Please describe.

Currently, the docs in the https://github.com/nginxinc/nginx-kubernetes-gateway/tree/main/docs folder are presented in a unordered way without any additional context, which makes it difficult to explore and navigate them.

Describe the solution you'd like
Add an index (or landing page) for the doc that list the ordered main pages and add some context about them, so that the readers can navigate the docs.

Describe alternatives you've considered
README. but I think it's better not to overload the README.

Additional context
Original suggestion from here #520 (comment)