[docs] How to override fetch

[gadicc]

Document how to ovverride fetch and when this might be useful (e.g. give a fetch-mock-cache example).

See commit https://github.com/gadicc/node-yahoo-finance2/commit/ea002e14ede0fbc61ceaed44c061f56e5ef79ef8.

Related: #923

Need to decide where to and how to best document all this but basically, as of now:

1. Without any overrides, we'll use globalThis.fetch at request time. Note: this differs from earlier behaviour where we'd save a ref during module load (v2) / instantiation time (<v3.6).

2. Specify a custom fetch at instantiation time.

   const yahooFinance = new YahooFinance({ fetch: customFetch })
   

3. Custom fetch after instantiation time (unstable API, don't rely on this)

   yahooFinance._env.fetch = customFetch; // Probably won't stay around
   

4. Custom fetch per request

   yahooFinance.quote(symbol, quoteOpts, { fetch: customFetch })