Clean up EntityMetaData and fix incorrect release tags

[pmconne]

Part of https://github.com/iTwin/itwinjs-backlog/issues/1103.

This was one of the very first APIs ever created for iTwin.js and boy does it show.
Despite still being tagged as @beta after all these years, it has always been de facto part of the @public API due to its use in @public methods like Entity.forEachProperty and Element.getClassMetaData. (api-extractor does not check ae-incompatible-release-tags across package boundaries).

I tried to do minimal cleanup without introducing breaking changes and add/clarify documentation. Mostly I preferred to link to the learning articles in docs/bis/ec; those need some separate cleanup, though.

[pmconne]

@khanaffan @ColinKerr @diegoalexdiaz please fact-check my documentation.

[ColinKerr]

Can we take a look at how this stuff is actually used before we promote to public?

[pmconne]

Can we take a look at how this stuff is actually used before we promote to public?

We can, but I want to reiterate: it is already public and has been for years, beta tag notwithstanding.
Booster's usage.

[mergify]

This pull request is now in conflicts. Could you fix it @pmconne? 🙏
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