Deals explainer (WICG#1237)

* Deals explainer

* Pluralize selectableBuyerAndSellerReportingIds

* beginnings of a table

* fill in table and use abreviations

* add line breaks

* improve language and text

* fixed problems

* fix typos

* add note about buyerReportingId in reportWin()'s browserSignals

* add more links to more details

* fix typos

* add note about matching list in IG

* add text about generateBid and scoreAd and examples

* update wording on k-anon rebidding

* fix typos and add another example

* A little clarification

* Resolve some of Orr's comments

* Merge in the last of Orr's edits

* Update FLEDGE.md

* remove old words about size

* clarify wording

* Address Orr's last comments