[DRAFT] Add HDR output manual page

[allenwp]

HDR output is planned for Godot 4.7. Along with its release, we should provide an new manual page describing how to use it. This PR is my attempt at this sort of a manual page.

There is still a lot of work to be done, but I wanted to share sooner rather than later to see if others agree or disagree with the structure, flow, and content that I have drafted.

You can try out the HDR output PR with my demo project to get a better understanding of the concepts shared in this manual page.

Notes to reviewers

• I'm not sure if this should be under Rendering or if it should have its own section, similar to XR. I think it's probably best as just one single page, so I've put it under Rendering.

[allenwp]

Maybe I should have a section dedicated to explaining how sRGB is used with HDR?

If I write about this, I should probably also write that Godot uses the Rec. 709 colour gamut (which is the colour gamut used by sRGB.)