docs: document requirements.txt auto-install — applies to all supported Indigo versions

[simons-plugins]

The skill shows bundling dependencies by running pip install -t "Contents/Packages/" manually. Indigo has a built-in auto-installer on all supported versions: place a requirements.txt in Contents/Server Plugin/ and Indigo installs the packages into Contents/Packages/ automatically at plugin startup. Never rely on system pip installs — they disappear on any Python upgrade.

The only version-specific difference is the success file name:

| Indigo         | Success file (in Contents/Packages/)          |
|----------------|-----------------------------------------------|
| 2025.2         | 3.13-pip-install-log-success.txt              |
| 2025.1 / older | pip-install-log-success.txt                   |

To force a re-install after editing requirements.txt: delete the success file, then restart the plugin.

Success file names confirmed from the filesystem of a live 2025.1 and 2025.2 installation.