docs: add missing step in docs for building WebView for Pi 5

[nicomiguelino]

Description

This pull request adds the missing step (where an environment variable named GIT_HASH should be set properly before building and starting the containers) in the documentation for building a WebView binary for Pi 5.

Checklist

• I have performed a self-review of my own code.
• New and existing unit tests pass locally and on CI with my changes.
• I have done an end-to-end test for Raspberry Pi devices.
• I have tested my changes for x86 devices.
• I added a documentation for the changes I have made (when necessary).

[vpetersson]

Can be simplified with:

GIT_HASH=$(git rev-parse --short HEAD) \
    docker compose -f docker-compose.pi5.yml up -d --build

Since we're not using this anywhere else.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud