Embroider: Port to v2 format: part 2: separate docs-app

[bantic]

Follow-up from #61. This adds a new pkg to the monorepo: the docs app, following the embroider guide section here: https://github.com/embroider-build/embroider/blob/main/docs/porting-addons-to-v2.md#part-2-optional-split-docs-from-tests

I'm opening for review. This is best reviewed by looking at the relative diff to #61: bantic/embroider-native...bantic/embroider-native-v2-docs-app

I'll rebase it after #61 is merged.

The tldr:

• Follows roughly the same process as in Embroider: Port to v2 format: Part 1: separate test-app #61, except this time we duplicate the test-app into docs-app and tailor each to have only the specific things (deps, templates, tests) it needs
• Adds a few very basic "does this render" tests for the docs-app
• Upgrades eslint to latest (v8 -> v9) for the test-app and docs-app, which requires creating a wholly new eslint config (details at https://eslint.org/docs/latest/use/migrate-to-9.0.0) -- the ember addon output repo uses eslint v9 so this is mostly just files copied from there
• Addresses some QUnit-related lint failures now bc the ember-addon-output includes the qunit plugin for eslint. This was mostly mechanical: change assert.ok -> assert.true, change assert.equal -> assert.strictEqual. And in one case change an assertion that was called from a conditional to be called uncondtionally
• Add the eslint-related devdeps to the addon/package.json. These should have been in Embroider: Port to v2 format: Part 1: separate test-app #61 but they weren't (and weren't noticed because linting continued to work due to yarn-classic's package hoisting)

[mixonic]

Not a blocker, but I am curious how we can more consistently leverage Addepar's own linting/style defaults in this kind of codebase without it being a headache.

[bantic]

how we can more consistently leverage Addepar's own linting/style defaults

Yeah -- I don't have a great answer here. I suppose an addepar-specific eslint plugin seems like it would be a good initial step.