Add Deployment planners documentation

[sudo87]

This PR adds documentation related to Deployment Planners in CloudStack.

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📚 Documentation preview 📚: https://cloudstack-documentation--594.org.readthedocs.build/en/594/

[sudo87]

@blueoragnutan docbuild

[sudo87]

@harikrishna-patnala please review this pr

[prashanthr2]

I would suggest replacing.

Deployment planners determine how and where instances are placed across clusters within a zone
with
Deployment planners determine how instances are allocated within a zone based on configuration settings.

As deployment planner itself can work at the pod level and also the actual deployment is on particular host. I think it is better to just say zone.

Except that everything else lgtm

[rajujith]

@prashanthr2 based on my discussion with @sudo87 offline, we decided the retain the original statement as it's the correct one as per the functionality.

[rajujith]

@blueoragnutan docbuild

[Copilot]

Pull Request Overview

This PR adds comprehensive documentation for Deployment Planners in CloudStack, explaining how instances are placed across clusters within a zone.

Key changes:

• New documentation file explaining deployment planner concepts and available planners (FirstFitPlanner, UserDispersingPlanner, UserConcentratedPodPlanner)
• Updated VM volume allocators documentation to reference deployment planners
• Added visual diagram illustrating the deployment planner workflow

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

File	Description
source/adminguide/deployment_planners.rst	New documentation file providing detailed explanations of deployment planners, their behaviors, and configuration options
source/adminguide/index.rst	Added deployment_planners to the table of contents under "Managing VM and Volume Allocation" section
source/adminguide/vm_volume_allocators.rst	Updated to clarify when host.capacityType.to.order.clusters applies and added reference to deployment planners
source/_static/images/deployment-planner-diagram.png	New diagram visualizing the deployment planner decision flow

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[sudo87]

@blueoragnutan docbuild

[sureshanaparti]

lgtm

[sureshanaparti]

@sudo87 can you target to 4.22 branch?

[sudo87]

@sudo87 can you target to 4.22 branch?

Sure @sureshanaparti, please review and merge.

[harikrishna-patnala]

LGTM