docs: use envsubst for quick-start examples

[omribz156]

Summary

• Switches quick-start example processing from a generated per-variable sed script to envsubst.
• Converts the YAML example templates from reStructuredText substitution tokens to shell-style variables, e.g. $NVIDIA_IPAM_REPOSITORY.
• Keeps the generated examples/processed output stable for the current docs/common/vars.rst values.

Addresses #238.

Verification

• PowerShell parity check that parses docs/common/vars.rst, renders all examples/templates/**/*.yaml shell variables, and compares the result to the checked-in examples/processed/**/*.yaml files: passed.
• git diff --check: passed; Git reported only local Windows LF-to-CRLF working-copy warnings.

I could not run make process-examples in this local Windows environment because make, bash, and envsubst are not available here.

This was implemented with Codex assistance, with the patch kept focused and manually reviewed before submission.

[github-actions]

Documentation preview

https://mellanox.github.io/network-operator-docs/review/pr-375