Add docs for the Renderer API

[ArmandPhilippot]

Description (required)

Adds a new reference page for the Renderer API:

• move some types from the Integration API to the new Renderer API (some have been reworded, not all)
• reuse a structure similar to the Adapter API to explain how to build a renderer
• clean up the Integration reference
  • SSRManifest.pageModule.renderers was removed in v6, the type was a leftover from fix(v6): update integrations reference with v6 changes #13164
  • we now describe twice serverEntrypoint/clientEntrypoint in the Renderer API, so we might not need a third description in the Integration API, and I'm not sure the React link was helpful? (ie. this was old content, maybe it was useful at that time)

Feedback welcome:

1. We don't want to show complex code snippets, but something that is enough to understand how this works. I tried to provide minimal code snippets based on the existing renderers but I'm not sure if this is too minimal or not.

2. It seems /en/reference/renderer-reference existed before, so I had to update the redirections. I'm not sure how Netlify handles them... maybe we want another slug for the page to avoid any conflicts?

3. I wonder if it would make sense to have a new sidebar group for integration-related APIs instead of lengthening the list in "Other development APIs"? Something like:
   

Related issues & labels (optional)

• Closes Document the renderer API #13199
• Suggested label: add new content

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[matthewp]

great explanation of these two things

[matthewp]

Definitely much better than what we had before. Great job!

[yanthomasdev]

LGTM! Amazing work @ArmandPhilippot!

[ArmandPhilippot]

If even Yan doesn't have nitpicks about the wording, I guess this one is ready to be shipped! Thank you both for the reviews!

I guess the sidebar reorganization (my third point in "Feedback welcome") can wait (NWTWWHB), especially since Fred wants to do some clean up around third-party guides. Perhaps he has other changes in mind.