Sort_by ≠ weight & weight set

# Bug Report

## Environment

Zola version: 0.19.2

## Expected Behavior
When I have `sort_by` for section set to anything that `weight`, setting page "weight` will be silently ignored, making me wonder why weight is not working.

I suggest that zola should issue warning in this case.

## Current Behavior
No warning, silently ignored

## Step to reproduce
