Add FlexSearch plugin with search functionality for Nikola sites

[dacog]

Hi,

This commit introduces a new FlexSearch plugin that provides search functionality for Nikola static sites.

The plugin generates a JSON file containing article data, which is used with FlexSearch to enable search capabilities on the site. It includes instructions on how to implement search results in a div or an overlay. Also, contains added configuration options in a sample conf.py.sample file which allows users to customize according to their needs.

Here is an example image of the search:

I hope this time I didn't forget anything.

Best,

Diego

[Kwpolska]

Thanks for contributing!