Link to tutorial and add platform notes to HDR output docs.

[allenwp]

What problem(s) does this PR solve?

• Fixes HDR output: No warning against using emissive with max linear value godot-docs#11913 for the class documentation.
• No documentation states that the maximum and reference luminance setters can only be used on certain platforms.
• No class documentation links to the HDR output manual page.
• DisplayServer.window_get_output_max_linear_value was not updated to mention the new Window.output_max_linear_value_changed signal. I think this was a simple oversight and not intentional.

How did you solve these problem(s) and why do it this way?

To resolve godotengine/godot-docs#11913, I used the wording from godotengine/godot-docs#11920.

Was AI used to create any part of this PR?

No

Are there any parts of this PR that you are uncertain of or require special attention from reviewers?

No

[Calinou]

Looks good to me.

[Repiteo]

Thanks!