docs: add Function Edit Page implementation plan and feature entry

Signed-off-by: Stanislav Jakuschevskij <sjakusch@redhat.com>

Author: twoGiants

docs/STYLEGUIDE.md

@@ -25,7 +25,8 @@
 
 ## CSS
 
-- **Relative units only**: Use `rem` and `em` for all sizing (widths, heights, margins, padding, font sizes). Never use `px`.
+- **PatternFly first**: Use PatternFly component props for all layout and spacing. Only fall back to custom CSS when PatternFly does not cover the need.
+- **Relative units only**: When custom CSS is necessary, use `rem` and `em` for all sizing. Never use `px`.
 
 ## OCP Plugin Styling Constraints