Improve Physics Interpolation pages

[tetrapod00]

Quick editing pass on the Physics interpolation pages to make various minor improvements and bugfixes:

• Reordered the index so that troubleshooting is still the last page.
• Removed commenting on the physics interpolation index page.
• Removed some "Spatial" node references.
• Link to project settings and properties in a few places.
• Fix RST headers. RST headers like --- and ^^^ do not have an inherent meaning, but are determined by first usage of each in a page. So a couple pages here weren't quite rendering right because ^^^ was used before ---, despite the apparent intention being that --- was h2 and ^^^ was h3.

[mhilbrunner]

Great work and good catches, quite a haul :) Thanks for the attention to detail, merged.