Update best practices for multi-cluster allocation

[roberthbailey]

Is your feature request related to a problem? Please describe.
https://agones.dev/site/docs/guides/best-practices/#allocate-across-clusters is out of date as compared to the more recent guidance we added to https://agones.dev/site/docs/advanced/multi-cluster-allocation/.

Describe the solution you'd like
Update the best practices with the current recommendation; remove Agones MCA from best practices.

Describe alternatives you've considered
n/a

Additional context
n/a