Update sub-readmes for getting started

[momuno]

Motivation and Context

1. Why is this change required? Confusion on getting-started abounds!
2. What problem does it solve? sub-readmes provided inconsistent basic getting-started info.
3. What scenario does it contribute to? attempting to remove confusion around getting started
4. If it fixes an open issue, please link to the issue here. Chat Copilot getting-started readme cleanup #344

Description

This removes duplicate or incomplete information from the sub-readmes in webapp/ and webapi/ on basic getting started. It redirects users to the main instruction readme, and also points to the learn docs getting started guide (which should be the ONLY duplicate of these instructions).

This change does NOT:

• attempt to update detailed development or debug information on the backend. That falls under a separate task.
• attempt to update the optional material. This pre-existing material has not been tested and any updates fall under a separate task.

Therefore, there is an added "under development" section in the sub-readmes to alert readers on potentially remaining out-of-date material.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄