docs/premium: document premium functionality

[SoggySaussages]

Documenting premium functionality for both the end-user and the self-hoster.

• Specific perks of premium with exact numerical values, and includes features not enabled on the public instance.
• Relevant call counter keys and names of constants within the source.
• Instructions on acquiring and redeeming premium slots
• Screenshot of redeemed slots to hopefully prevent "where are my slots" "just scroll down"
• Details on sourcing premium slots for self hosters
• Brief description on changing limits within the source

Hosted as always on https://yagdocsv2.vedamaharaj.ca/yagpdb-docs-v2/premium.html

Signed-off-by: SoggySaussages vmdmaharaj@gmail.com

Terms

• I have read and understood this project's Contributing Guidelines

[l-zeuch]

Thanks!