Update index.md to mention that govuk-frontend javascript will auto-focus

[edwardhorsford]

The guidance makes it clear that the errorSummary should be focused on page load, but doesn't mention that this should happen for free with the govuk-frontend javascript.

Hopefully this makes it clearer so teams don't spend time trying to re-impliment this.

See similar PR on the NHS Service manual

Fixes #3818

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[owenatgov]

Thanks for this Ed. There are a few examples of this where we tell a user to do something when, if govuk is imported properly, it'll just happen. There's a bigger piece of work to remove all of these but for now I wonder if the solution is to just remove this line entirely. Maybe add something saying what the component will do. What do you reckon @edwardhorsford?

[edwardhorsford]

Thanks for this Ed. There are a few examples of this where we tell a user to do something when, if govuk is imported properly, it'll just happen. There's a bigger piece of work to remove all of these but for now I wonder if the solution is to just remove this line entirely. Maybe add something saying what the component will do. What do you reckon @edwardhorsford?

If you remove the line, I'd make sure to note somewhere that the element should be focused on page load - and that the js should do that for you. I think focusing is disabled in the examples, and if a team were reimplementing they might not know.