Update package metadata readme to a docs page instead of a change record

Problem/Motivation

\Drupal\Composer\Generator\ComponentGenerator::initialPackageMetadata adds a _readme section to the composer.json file of generated packages that points to a change record, but it should point to a docs page.

Steps to reproduce

Proposed resolution

• Create a docs page,
• Update ComponentGenerator to link to that docs page
• Ensure all generated packages have the updated link

Remaining tasks

All of it

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet