Improve the overview section of the user guide

[teeceeas]

As a FMD user,
I want the overview section of the user guide to provide a clear and concise introduction to the service, its purpose, and how it can help me,
So that I can quickly understand its value and navigate the catalogue effectively.

Acceptance Criteria:

• Indicate what content is on the catalogue
• Pair to build the content with @sdurnberger
• Include simple diagrams and simplify the information about how data is ingested etc
  Eg. https://docs.sign-in.service.gov.uk/how-gov-uk-one-login-works/#understand-the-technical-flow-gov-uk-one-login-uses

[DeanWillicombe82]

Requires input from @jemnery

[sdurnberger]

Currently using this doc in our shared folder for initial drafting, pairing with @murdo-moj tomorrow 👍

[DeanWillicombe82]

Chased @sdurnberger for an update.

[DeanWillicombe82]

Update from @sdurnberger - Having issues with Github at present. Hoping to have a PR up for this tomorrow 07/03