Boost: documenting the filters used in the plugin

[donnchawp]

The existing documentation covering the filters used in the plugin are woefully out of date.

Fixes #36250

Proposed changes:

• Adding docs to docs/DEVELOPEMENT_GUIDE.md

Other information:

• [ ] Have you written new tests for your changes, if applicable?
• [ ] Have you checked the E2E test CI results, and verified that your changes do not break them?
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Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

No testing required, but needs to be read and verified.

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[donnchawp]

It looks like jetpack_boost_module_enabled has been completely removed. It only exists now in tests and the documentation.

[pyronaur]

@thingalon seeing is believing 😅 - the guide has fallen out of fashion multiple times. So yeah, I agree - documentation should probably go in code comments.

[haqadn]

Is this ready for another review/merge @donnchawp ?

[donnchawp]

@haqadn no, I decided other PRs were more urgent and put this aside for now. Should I close it?

[haqadn]

Since this is almost ready, lets finish this. @donnchawp