Add hnwlib.rb and ngt-ruby gems to Vector search

[yoshoku]

I would like to add the hnswlib.rb and ngt-ruby gems to the Vector search section. They are ruby bindings for vector search with graph structure. In addition, I relocated the flann and annoy-rb gems to the Vector search section from the Clustering section. Since they provide vector search functions with tree structure, they are better listed in the Vector search section rather than the Clustering section.

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I want to contribute to this list and help the maintainers, so:

• [x] I've accepted the terms of the CC0 License;
• [x] I've added the topic rubyml to the contributed repository if appropriate.

[arbox]

@yoshoku, appreciate your interest to contribute to this list!