Add examples for run_model functions in v0.15.1

[Chirag3841]

Added examples for run_model_from_poa() and run_model_from_effective_irradiance() in the documentation.

• [ x] Closes Add examples for run_model methods, add docstrings for ModelChain constants #1043
• [x ] I am familiar with the contributing guidelines
• Tests added
• [x ] Updates entries in docs/sphinx/source/reference for API changes.
• [x ] Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes. Includes link to the GitHub Issue with :issue:`num` or this Pull Request with :pull:`num`. Includes contributor name and/or GitHub username (link with :ghuser:`user`).
• [ x] New code is fully documented. Includes numpydoc compliant docstrings, examples, and comments where necessary.
• [ x] Pull request is nearly complete and ready for detailed review.
• Maintainer: Appropriate GitHub Labels (including remote-data) and Milestone are assigned to the Pull Request and linked Issue.

[cwhanse]

Please add this in #2621 not in a new PR.