[docs] Align pack name convention documentation with schema

[brondani]

Fixes

• https://github.com/Arm-Debug/devtools-external/pull/1369

Changes

• pack vendor is mandatory

Checklist

• 🤖 This change is covered by unit tests (if applicable).
• 🤹 Manual testing has been performed (if necessary).
• 🛡️ Security impacts have been considered (if relevant).
• 📖 Documentation updates are complete (if required).
• 🧠 Third-party dependencies and TPIP updated (if required).

[jkrech]

LGTM - I assume we now document what is implemented and if we wanted to make the pack vendor optional we would need to request that as a new feature.