[DOC beta] Octanify documentation

[locks]

Part of #18250.

Missing

• what to do with mut in get documentation
• bring back classic component example for component helper
• normalize set and tracked in computed_macros file
• review each-in for correctness/coesion (key is used before it's introduced)
• confirm get + mut example updates properly
• action_handler switch examples to EmberComponent so that we can still use the classic .extend API.
• investigate syntax highlighting mishaps - hasBlock, hasBlockParams
• observer is Helper class example
• RouterService#recognize uses a strange ember component

Missing code samples

• RouterService#isActive
• RouterService#recognizeAndLoad

Open questions

• classes, named or anonymous? (class extends Route vs class Menu extends Route) -> named

Router Service

• constructor seems ok in code samples? (cc @pzuraq)
• currentRoute example is using notEmpty CP decorator. should we replace with native getter? (cc @pzuraq)

unless

• Consider introducing not to the framework, and deprecating unless for {{if (not)}}

Action Handler

• (?) action_handler remove mixins (?)

Controller

• figure out what to do with Controller#addObserver - it's currently using an EmberComponent, which seems strange

action helper

• figure out how we want to address classic EmberComponent in action helper

partial

While the partial deprecation is not currently implemented, it is possible that Octane will ship with it.
Regardless, we should definitely suggest GlimmerComponents instead.

Helper (class)

• @observer?

Route

• on('activate', function(){ + native class?
• willTransition, didTransition, loading, error actions + native class?

[rwjblue]

I don't fully understand what the plan here is. Is there any reason we shouldn't land these fixes independently (vs wait for everything to be done in this checklist).

[locks]

@rwjblue no reason, I was thinking that we could use this branch to review things before sending them off to master, but we can do that off of master and spare the indirection. I'm updating the instructions and my PR in that direction.

[locks]

Closing this as I'm splitting it into multiple PRs.