Add guidance for tables with lots of data

[owenatgov]

What

Add a section to the bottom of the tables page with instructions on how and when to use the small text from tablet modifier introduced in alphagov/govuk-frontend#4630.

Closes alphagov/govuk-frontend#3922

Important

This change can only be merged after the new typography scale is launched as part of #3456

Why

So that we are advertising that this feature exists to users as well as adding some light guidance on minimising data in a table where possible.

Notes

I need to make it clear that this is currently only tied to 5.2 if you turn the feature flag on. This could link to the new scale guide that now lives in #3343.

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[owenatgov]

I'm pointing this at the typography-guidance branch because it means we can tackle the issue of illustrating that this is only for if you have the feature flag set to on holistically.