Update user docs for Central v2026.3

[matthew-white]

• Upgrade instructions
• Merge Web Forms docs PRs before release
• Changes to entity CSV upload
  • Smooth out entity CSV upload experience central#1784
• Custom properties on projects
  • Remove "custom properties" page central#2075
  • Inline creation of user properties from the App User / PAL create modals central#1875
  • Add "View as [app user]" filter to Entities central#1792

[matthew-white]

I'm going to go ahead and close now that there look to be PRs up for the changes listed above. We'll be releasing shortly.