docs: make Quickstart vendor-neutral using helm create

[Iam-Karan-Suresh]

Summary

This PR updates the Helm Quickstart Guide to avoid relying on Bitnami charts
and classic Helm repositories in the main onboarding flow.

The guide now demonstrates Helm fundamentals using a locally generated chart
via helm create, while still acknowledging third-party repositories and
OCI-based charts as valid options.

Motivation

Bitnami has announced upcoming catalog and image changes, which may require
additional configuration (such as legacy image overrides) and can be confusing
for new users following the Quickstart.

Since the Quickstart is intended to be a stable, beginner-friendly introduction
to Helm, depending on a specific third-party vendor introduces unnecessary
fragility.

What changed

• Removed Bitnami-specific commands from the main Quickstart flow
• Replaced the example with a vendor-neutral helm create → helm install workflow
• Added a short note acknowledging Bitnami and other third-party repositories
• Clarified that Helm supports both local charts and OCI-based registries

Why helm create

Using helm create:

• Avoids external dependencies
• Works consistently in all environments
• Teaches core Helm concepts (charts, installs, releases)
• Aligns with the suggestion raised in the original issue discussion

Scope

This is a documentation-only change and does not modify Helm behavior.

Feedback and suggestions are welcome.
issue: #1752