Add PWA link to unstable versions of the Configurator on README

[McGiverGim]

Add a link to the PWA version of the Configurator, under "Unstable Testing Versions".

Is only a temporary link but can help to test things, until we move to PWA really. Then we will need to redo the README.

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[nerdCopter]

The support section around line 151 can be updated as well. replace Slack info with Discord info.

[McGiverGim]

The support section around line 151 can be updated as well. replace Slack info with Discord info.

I've replaced it with the same text than we have in the firmware README.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarCloud

[github-actions]

Do you want to test this code? Here you have an automated build:
Betaflight-Configurator-Android
Betaflight-Configurator-Linux
Betaflight-Configurator-Windows
Betaflight-Configurator-macOS
WARNING: It may be unstable and result in corrupted configurations or data loss. Use only for testing!