docs(breaking): update stencil v3.0.0 breaking changes (#3691)

this commit updates `BREAKING_CHANGES.md` by adding a table of contents
to the document, as well as an executive summary. the goal of this
commit is to align with our sibling project, the Ionic Framework, it
terms of layout of the document.

keeping with that philosophy, existing header tags have been updated to
be one level 'smaller' to accommodate the new structure

this commit updates the `BREAKING_CHANGES.md` document to include all
breaking changes made to date in the stencil v3.0.0-dev branch