Add "alternative words" to filter items in docs

[hikalkan]

For example, user may type "Error" instead of "Exception". In that case, it would be good to also find the "Exception" in filter results.

We can simply add optional "keywords" definition to an item on the navigation tree.

Example:

{
  "text": "Exception Handling",
  "path": "framework/fundamentals/exception-handling.md",
  "keywords": ["Error", "Another Value"]
}

We may define more than one keyword in the array as shown above. Then "filtering" will also consider these keywords.

This will be done in the docs module.