Add docs about using StaticLookup

[sacha-l]

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Content request

We should add docs about using StaticLookup instead of the AccountId type.

There's already a lot of useful information here: https://substrate.stackexchange.com/questions/4339/tlookup-as-staticlookupsource-vs-taccountid

And this PR gives good context too.

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Maybe (please elaborate above)