update solochain to use #[runtime]

[Jan-Jan]

Description

• This is part of issue 5242, specifically getting solochain to use #[frame::runtime]
• Furthermore, reinforced the convention of Template instead of TemplateModule

Integration

• Should be integrated into the solochain template and documentation

Review Notes

• Refactored solochain template from construct_runtime! to #[runtime].
• AFAIU Template is our new convention, and preferred over TemplateModule.

Out of scope

• The #[runtime] documentation is still very rudimentary, and should ideally be expanded to explain the macro, both what it does and the input options.
• Furthermore, suggest update #[runtime] documentation to replace #[crate::runtime] with #[frame_support::runtime]