[improve docs] Example pallet crate and Basic Example pallet (#1546)

This fixes the broken links in the crate level documentation of the
Examples crate. It also updates the documentation for the Basic Example
pallet by removing the template for documenting a pallet (we now have
[this](https://github.com/paritytech/polkadot-sdk/blob/master/docs/DOCUMENTATION_GUIDELINE.md)
to refer to instead).

Note: I found it unnecessary to provide a link to the doc guidelines as
I don't think this would be where someone should discover them. I also
want to flag some ideas that came while making these minor improvements
in [this
issue](https://github.com/paritytech/polkadot-sdk-docs/issues/27) (for a
subsequent PR) as part of ongoing docs work.