doc and sample for xtk:builder#InstallPackage method

[mkiki]

Description

Add documentation for the InstallPackage method call which allows to install a package. Documentation is useful because the structure of the expected XML document is not obvious.

Motivation and Context

Questions from users

How Has This Been Tested?

Regenerated the documentation locally, ran sample on local instance and checked results manually.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] Documentation and samples update

Checklist:

• [x] I have signed the Adobe Open Source CLA.
• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.