Docs: Clarify how networking between data plane propeller and control plane data catalog can be configured in multi-cluster deployment

[fg91]

Why are the changes needed?

The multi-cluster deployment documentation is slightly ambiguous on how the networking between the data plane cluster flytepropeller service and control plane cluster datacatalog service works, suggesting that datacatalog was exposed via the ingress.

I clarify that the user would need to expose datacatalog themselves and that datacatalog does not have its own auth mechanism. I also suggest to use a VPC-internal load balancer service for this purpose instead.

• [x] I updated the documentation accordingly.
• [x] All commits are signed-off.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 61.10%. Comparing base (c0f5b10) to head (62c0645). Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5345      +/-   ##
==========================================
- Coverage   61.10%   61.10%   -0.01%     
==========================================
  Files         794      794              
  Lines       51213    51213              
==========================================
- Hits        31295    31294       -1     
- Misses      17037    17038       +1     
  Partials     2881     2881              

Flag	Coverage Δ
unittests-datacatalog	69.31% <ø> (ø)
unittests-flyteadmin	58.90% <ø> (ø)
unittests-flytecopilot	17.79% <ø> (ø)
unittests-flytectl	68.30% <ø> (ø)
unittests-flyteidl	79.30% <ø> (ø)
unittests-flyteplugins	61.94% <ø> (ø)
unittests-flytepropeller	57.32% <ø> (ø)
unittests-flytestdlib	65.73% <ø> (-0.03%)	:arrow_down:

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