Performance control of embedded content (#911)

* Create explainer.md

* Nits

* Migrate and expand on sections 5.1, 5.2, 5.3

* Add acknowledgements section

* Nit

Added comma

* Fixed typo

* Fix typo

* Re-order non-goals section

* Add 'remove burden' to goals

* Re-arrange sections

* Category list spacing

* Category list spacing

* Category list formatting

* add internal section link: negotiation

* Add link to header docs

* Add single category example

* Grammar fixes

* Re-word alternatives considered

* Grammar and formatting edits.

* Acknowledgements & spelling

* Expand on use cases and scenarios section

* Remove concrete numbers for limits & fix typos in proposal table

* Add more context about what to do with reports in the intro section

* Add note in proposed solution about source of proposal

* Clean up description of basic category

* Add context to "what should be standardized" section

* Clarify non-goals section

* Clarify the different script categories

* Fix formatting

* Added open questions

* Nit

* Nit

* Re-work 'best practices' section

* Merge Proposed Solution/API Design Details

* Small edit in goals section

* Define categories and criteria early on

* Added more details to the problem we are trying to solve.

* Update README

* Add group in README

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Co-authored-by: Luis Flores <[email protected]>
Co-authored-by: Andy Luhrs <[email protected]>