feat: configure Devtron probes via Helm (#6964)

[sagarkt]

Description

Expose the main Devtron deployment's liveness and readiness probe timing fields through Helm values so installations with slower startup behavior no longer need a post-install patch.

• adds components.devtron.livenessProbe and components.devtron.readinessProbe
• preserves the current timing defaults and /health handler
• documents a slow-start override example in the chart README

This provides a clean, validated implementation of the feature also attempted in the existing unreviewed draft #6965.

Fixes #6964

How Has This Been Tested?

• helm lint against the first-party chart templates (remote dependencies omitted)
• helm template --show-only templates/devtron.yaml with default values
• helm template --show-only templates/devtron.yaml with independent liveness and readiness overrides

Checklist:

• The title of the PR states what changed and the related issues number (used for the release note).
• Does this PR requires documentation updates?
• I've updated documentation as required by this PR.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have tested it for all user roles. (Not applicable to a rendered Deployment setting.)
• I have added all the required unit/api test cases. (No unit/API behavior changes; Helm rendering was validated.)

Does this PR introduce a user-facing change?

The `devtron-operator` Helm chart now supports `components.devtron.livenessProbe` and `components.devtron.readinessProbe` timing overrides for the main Devtron deployment.

[bito-code-review]

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