Update boilerplate text for collections

[ariordan-redhat]

Summary

Reword the boilerplate intro to collection modules so they don't suggest a user has to install the collection.

Issue

The current wording suggests the user should install collections:

This plugin is part of the <collection_name>.
To install it, use: ansible-galaxy collection install <collection_name>.

This is misleading, as all the collections are included in the Ansible package.

Notes

Mentioned in Documentation Working group (DaWGs )meeting 9/21/2021.

Affects:

• antsibull/data/docsite/role.rst.j2
• antsibull/data/docsite/plugin.rst.j2

[felixfontein]

Fixed in #318.