Update in-codebase documentation for E2E tests

[Trenly]

📖 Description

Adds documentation for some E2E test settings that was missing

🔗 References

🔍 Validation

Copilot source code review

✅ Checklist

• Signed the Contributor License Agreement
• Linked to an issue
• Updated Release Notes (if applicable)
• Updated documentation (if applicable)
• Updated Copilot instructions (if build, architecture, or conventions changed)

📋 Issue Type

• Bug fix
• Feature
• Task

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

Updates the in-repo E2E test documentation and defaults so developers (and automation) have a clearer, more complete set of TestRunParameters for AppInstallerCLIE2ETests.

Changes:

• Expand Test.runsettings to include additional E2E parameters (e.g., local server cert, in-proc testbed, forced experimental features).
• Update the E2E README to document the added parameters and show them in the example configuration.
• Add a Copilot instruction file to keep TestSetup.cs, README.md, and Test.runsettings parameter docs in sync, and ignore developer-local .runsettings files by default.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/AppInstallerCLIE2ETests/Test.runsettings	Adds/renames documented E2E run parameters and updates default parameter entries.
src/AppInstallerCLIE2ETests/README.md	Documents additional E2E runsettings parameters and updates the example snippet.
.gitignore	Ignores developer-specific *.runsettings while keeping the tracked example runsettings file.
.github/instructions/e2e-test-setup-parameters.instructions.md	Adds guidance to update parameter docs in README and Test.runsettings when modifying TestSetup.cs.