Expanded documentation for vector slide and project

[theraot]

This pull request expands the documentation of Vector2.slide, Vector2.project, Vector3.slide and Vector3.project.

Based on godotengine/godot-proposals#8173 and godotengine/godot-docs#8286

Fixes godotengine/godot-docs#8286

[esklarski]

I like it. It's clearer than my initial suggestion.

[kleonc]

Checked correctness, LGTM. 👍 Left some minor suggestions.

Also C# docs in Vector2.cs/Vector3.cs would need to be synced.

[theraot]

I have updated Vector2.cs and Vector3.cs.

[Mickeon]

Great to have. If not for this PR, I would've gotten around writing something like this when it was time.

[akien-mga]

Thanks!