Update description of shader variable TIME and add description to Fog, Particle, Sky, and Spatial shader pages

[tetrapod00]

Added/changed description of TIME in CanvasItem, Fog, Particle, Sky, and Spatial shader pages. Description now links to other relevant docs pages. Added a label to the global uniform header of shading_languge.rst, so it can be cross referenced from elsewhere.

I made some changes in addition to simply copying the existing description from CanvasItem shaders to the other pages. Should close #9725.

I wrote for a user who is new-ish to shaders, notices that TIME does not exactly match CPU time behavior, and finds any of the shader pages. Ideally they can come away the exact behavior of the built-in TIME uniform, as well as a way to make their own if need be. I based this on my interactions with users in various discords who asked about TIME.

New description which appears on each shader page:

Global time since the engine has started, in seconds. It repeats after every 3,600 seconds (which can be changed with the rollover setting). It's not affected by time_scale or pausing. If you need a TIME variable that can be scaled or paused, add your own global shader uniform and update it each frame.

Old description, from the CanvasItem shader page:

Global time since the engine has started, in seconds (always positive). It's subject to the rollover setting (which is 3,600 seconds by default). It's not affected by time_scale or pausing, but you can define a global shader uniform to add a "scaled" TIME variable if desired.

Changes:

• Now links to the rollover setting in project settings. This is important, because searching for "rollover" in the docs does not actually surface the setting in ProjectSettings, only this page.
• Now links to the global shader uniform documentation. A label is added to the global uniform header in that docs page. I may have gotten the preferred syntax wrong for this.
• Uses more active language for the sentence about adding your own TIME uniform.
• Clarifies that the value repeats rather than merely "being subject to" the rollover setting.
• Emphasizes the default value of 3,600 seconds first, because a user finding this page is unlikely to have changed that value.
• No longer mentions that the value is "always positive". If this is necessary, I would suggest "always in the range of 0 to 3,600" or "always in the range of 0 to rollover".

If any of these changes break docs style, or are otherwise too much, I can revert to more minimal changes.

Also sent in godotengine/godot#95381, so that the link to the setting from the shader docs page doesn't point to an empty description.

[clayjohn]

Looks great to me! Good work

[skyace65]

Thanks!