Discord: Add support for Mentions

[thijsvanloef]

Context

Fixes https://github.com/thijsvanloef/palworld-server-docker/issues/300

Changing the way the Discord messages are built, Allowing you to add a user ID to the discord message to mention people or roles.

Choices

Move the message from the Title to the description and add the script name as a title.

Test instructions

1. Build the container
2. Set DISCORD_WEBHOOK_URL and change any of the MESSAGES (e.g. DISCORD_PRE_START_MESSAGE) to include a mention using <@user_id>
3. Check if you are mentioned in the Discord Channel

Checklist before requesting a review

• [x] I have performed a self-review of my code
• [x] I've added documentation about this change to the README.
• [x] I've not introduced breaking changes.