Docs: update all documentation

[katie-gardner]

Overview

Ran through all the documentation looking for anything outdated or missing and updated anything I found

Also removed some legacy migration scripts which have since been replaced with the export-processor and contentful-managment import process

Changes

• Since first implementing redis we've improved the process so you no longer need to manually cache each contentful usage - so I've removed this from Conventions.MD
• The seed test database, isn't really being used the way it was first created, but is now a great way to have a local test database, so I've updated docs accordingly, renaming the project could be done to, depending on the plan for its future usage
• Removed a couple of other sentences referring to content in the database, which is no longer the case
• Added a couple of additional links to help onboarding devs find the relevant READMEs more easily

How to review the PR

Check that all the updated documentation is correct and makes sense

Checklist

Delete any rows that do not apply to the PR.

• Title uses Angular commit convention
• PR targets development branch
• Documentation has been updated where relevant

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud