Refactor README to be more streamlined and reflect status today

What does this MR do?

MR refactors the README along with some other docs additions as follows:

• Removes direction section with any parts still relevant now contained elsewhere in the README.
• Adjusts the tone about the Toolkit to match the reality today and call out that it's a set of tools to help enable users to deploy GitLab at scale instead of a one click solution (as it could've been construed)
• Adds a callout for expected base knowledge for the Toolkit, highlighting more managed offerings like SaaS or Dedicated also.
• Adds a note about GitLab KAS and the need to configure it via custom Config.
• Various lint fixes

Author's checklist

When ready for review, the Author applies the workflowready for review label and mention @gl-quality/get-maintainers:

• Merge request:
  • Corresponding Issue raised and reviewed by the GET maintainers team.
  • Merge Request Title and Description are up-to-date, accurate, and descriptive
  • MR targeting the appropriate branch
  • MR has a green pipeline
  • MR has no new security alerts in the widget from the Secret Detection and IaC Scan (SAST) jobs.
• Code:
  • Check the area changed works as expected. Consider testing it in different environment sizes (1k,3k,10k,etc.).
  • Documentation created/updated in the same MR.
  • If this MR adds an optional configuration - check that all permutations continue to work.
  • For Terraform changes: set up a previous version environment, then run a terraform plan with your new changes and ensure nothing will be destroyed. If anything will be destroyed and this can't be avoided please add a comment to the current MR.
• Create any follow-up issue(s) to support the new feature across other supported cloud providers or advanced configurations. Create 1 issue for each provider/configuration. Contact the Quality Enablement team if unsure.