docs: explain launcher mod installation

Summary

Adds a launcher-first path to the mod-installation guide for users who do not want to edit openmw.cfg by hand.

• documents Data Files → Data Directories → Append;
• explains optional detected subdirectories and directory ordering with Move Up / Move Down;
• retains the existing manual configuration path and plugin-enabling steps.

Addresses #9300.

Validation

• git diff --check
• sphinx-build docs/source docs/build

A clean baseline build and this branch both completed with the same inherited 75 documentation warnings; this change added no warnings.