Improve custom loader documentation

[robwalch]

This PR will...

Improve custom loader documentation

Why is this Pull Request needed?

Resolve recent loader issues:

• How to composite ts files encrypted with AES-256bit with a custom loader and pass them to hls.js. #7007
• Is it possible to configure the loader to intercept the remote request of the manifest and return the locally cached m3u8 content? #7012
• How to write my own loader? #7010
• Pass Decrypted Data back to HLS.js using a custom loader #7022

Are there any points in the code the reviewer needs to double check?

Recent issues have been labelled "Custom loader":
https://github.com/video-dev/hls.js/issues?q=is%3Aissue%20state%3Aopen%20label%3A%22Custom%20loader%22

TODOs:

• Add "Custom loader" label to older issues
• Identify implementation sticking points for all issues and address in docs
• Response filter example (Pass Decrypted Data back to HLS.js using a custom loader #7022)

1. Handle specific types of requests (playlists, fragments, other): 7fec594
   a. config usage
   a. loader context usage
2. Providing loader stats (type structure and dependencies)
3. Extending existing loaders with composition vs inheritance (example covers inheritance and facade patterns)

Resolves issues:

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md

[itsjamie]

Thanks Rob.