docs: Add Template Tag format guides

[IgnaceMaes]

While eventually all guide material will have to be updated to use the <template> tag authoring format, this PR serves as a first introduction to the format. This unblocks moving forward emberjs/rfcs#871.

Notes:

• The syntax highlighter used (Prism) doesn't support glimmer-js nor glimmer-ts. I don't think it should block introducing the concept of the format here. But it does seem like a blocker to integrate the format in other parts of the guides.

Sources:

• https://rfcs.emberjs.com/id/0779-first-class-component-templates/
• https://github.com/ember-template-imports/ember-template-imports
• https://ignacemaes.com/emberconf-2023-template-tag-slides/

To review

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Review for:

• Typos
• Mistakes
• Completeness

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[ef4]

One of the biggest teaching points I think we need to hit here is how to locate the components/helpers/modifiers you're trying to import. There are generally-applicable, simple rules that work for essentially all addons. For example, an addon allowing you to importi a component from your-app-name/components/the-component-name is effectively mandatory.

[ef4]

Another gotcha to document: to use an imported template, it needs to not be using the old, pre-template-colocation format where the templates is in app/templates/components with the JS in app/components. People should ideally codemod off that format before trying to use template tag.

If you must use it, you can use setComponentTemplate manually to stick the template and JS together, but probably that is not guides-level content.

[IgnaceMaes]

Thanks @NullVoxPopuli @ef4 for the feedback.

I did a full rewrite from scratch instead of starting from the ember-template-imports README. It's restructured beginning at feature parity with the current format, only to introduce new capabilities afterwards. That makes most sense to me.

Marked as ready for review!

[evoactivity]

Just a few typos, rest looks good to me

[mansona]

The learning team did a review on this PR. These are minor changes but I think they should help get this perfectly ready to merge and launch 🎉 Let me know if you have any questions about my comments 👍

[IgnaceMaes]

All comments should be addressed now, thanks for the great input!

I've added another short section on importing conventions for helpers and modifiers. They change from kebab-case to camelCase as it now follows ES import standards. I've seen some confusion and questions on Discord about this so I believe this had to be addressed as well.

[mansona]

LGTM 💪 🎉 🚀