docs: Add topic on creating custom managers in galasa.dev site

[eamansour]

Why?

Refer to galasa-dev/projectmanagement#1410

Adds a new documentation page covering how to scaffold a custom Galasa manager project using galasactl project create --manager.

Changes

• New page: docs/content/docs/cli-command-reference/creating-a-manager-project.md
  • Explains what a Galasa manager is and when to write one
  • Documents flags for galasactl project create --manager
  • Shows the full generated folder and file structure
  • Describes each generated Java class and its role
  • Documents the manager lifecycle methods that are currently implemented in project create (initialise, youAreRequired, provisionGenerate, provisionDiscard)
  • Covers building the project with Maven and Gradle
  • Shows how to add the manager as a dependency in a test project
• docs/content/docs/cli-command-reference/index.md: Adds a link to the new page in the CLI command reference index
• docs/mkdocs.yml: Adds the new page to the site navigation under CLI command reference

[jadecarino]

One minor comment but happy to approve - thanks for adding these!