Start simplifying the 'Getting started' docs and use collapsible blocks for optional content

[eamansour]

Why?

Refer to galasa-dev/projectmanagement#2581

Changes

• Started simplifying the content on the following pages:
  • "Initialising your local environment"
  • "Installing the Galasa CLI"
  • "Running a test locally" (note: we still need to separate the Gherkin-related content from this page out into a separate topic as we don't cover writing Gherkin tests anywhere)
• Moved "Setting the Galasa home folder (optional)" into a collapsible block
• Moved "Overriding the path to the default local Maven repository" into a collapsible block (since this is optional and users can just use the default .m2 repo)
• Removed the "Setting the Galasa bootstrap" section from the "Initialising your local environment" page since this focuses on setting GALASA_BOOTSTRAP to an ecosystem's bootstrap endpoint (which is not something users would do when first getting started with Galasa)
• Documentation updates (if applicable)

[jadecarino]

Two minor suggestions but otherwise I've had a look at the site with changes locally and looks great!