Update documentation on settings validation in response to code changes

[ThomasHepworth]

Type of PR

• [ ] BUG
• [ ] FEAT
• [ ] MAINT
• [x] DOC

Is your Pull Request linked to an existing Issue or Pull Request?

• Recent changes updating the code base - https://github.com/moj-analytical-services/splink/pull/1764
• Initial documentation PR - https://github.com/moj-analytical-services/splink/pull/1648

Give a brief description for the solution you have provided

Updates the settings validation documentation following some fundamental changes to the way it is coded.

The code has largely moved towards a more functional approach, simplify the process of extending it. As a result, a lot of the documentation has been stripped back and simplified.

PR Checklist

• [x] Added documentation for changes
• [ ] Added feature to example notebooks or tutorial (if appropriate)
• [ ] Added tests (if appropriate)
• [ ] Updated CHANGELOG.md (if appropriate)
• [ ] Made changes based off the latest version of Splink
• [ ] Run the linter
• [x] Run the spellchecker (if appropriate)