Update README information around toggles usage in Simorgh

[tochwill]

Is your feature request related to a problem? Please describe.

• We need to update the README in useToggles as it's outdated and says toggles aren't service specific, they now can be in iSite.
• We should add an explanation of how remote toggles and default toggles work in the README that exists alongside the toggles config. This should explain how and where the data is fetched and merged, and the function of the default toggles.
• localToggles in src/app/lib/utilities/getToggles/index.js should be renamed to defaultToggles (or similar) for clarity.

Describe the solution you'd like
Make the required README updates to improve clarity over how Simorgh toggles work.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Testing notes
[Tester to complete]

Dev insight: Will Cypress tests be required or are unit tests sufficient? Will there be any potential regression? etc

• This feature is expected to need manual testing.

Additional context
Add any other context or screenshots about the feature request here.

[andrewscfc]

Readme is still wrong and needs updating: https://github.com/bbc/simorgh/blob/latest/src/app/hooks/useToggle/README.md