Pallet view functions: improve metadata, API docs and testing (#7412)

- [x] refactor view functions metadata according to #6833 in preparation
for V16, and move them to pallet-level metadata
- [x] add `view_functions_experimental` macro to `pallet_macros` with
API docs
- [x] improve pallet-level UI testing for view functions

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Co-authored-by: cmd[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Guillaume Thiolliere <gui.thiolliere@gmail.com>