Add docs page for new `TavilyFetcher` component

[sjrl]

Add a docs page for the TavilyFetcher component added in deepset-ai/haystack-core-integrations#3561

It should be added to the fetchers section https://github.com/deepset-ai/haystack/tree/main/docs-website/docs/pipeline-components/fetchers

It should also be added to the 2.31.0 versioned docs

[prnv19]

Hi, I'd like to work on this. I'll add the TavilyFetcher docs page under the fetchers section following the existing pattern in that directory, and include it in the 2.31.0 versioned docs as noted. Let me know if there's a specific fetcher doc page I should mirror the structure from, otherwise I'll base it on the closest existing one.