Add erratum to 1.0 and backwards compatibility statement to 1.1.1

[mattgarrish]

This pull request updates the errata page for the EPUB Accessibility 1.0 specification to add an entry making the inclusion of an accessibilitySummary a recommendation instead of a requirement. It also notes that repeating information in the other metadata is not necessary, but this was never a requirement so publications that did repeat the info aren't non-conforming.

(The errata page will be published through a different process than the W3C specs.)

I've also re-integrated the changes from pull request #2527 that add a section on backwards compatibility.

Fixes #2526

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Accessibility 1.0 Errata: Preview Accessibility 1.1.1: Preview | Diff

[mattgarrish]

For reference, I copied the formatting of the errata from the content documents page: https://idpf.org/epub/301/spec/epub-contentdocs-errata/

[mattgarrish]

Hm, I just remembered that we switched the requirements for access mode and access mode sufficient in 1.2.

That means that 1.2 content isn't backwards compatible, although it's not critical because each version has its own identifiers. But it means I need to amend this PR to take out the proposed section on compatibility.