Add inline code examples in data processing module docstrings

As first discussed in [this comment](https://github.com/GenericMappingTools/pygmt/issues/1666#issuecomment-993962742), module docstrings should contain inline code examples to show a working example of the module with the correct syntax without needing to find an example page.

To prevent example functions from being run in tests, add `_doctest_skip__ = [MODULE NAME]` under the import statements.

- [ ] binstats
- [x] blockmean #1729
- [x] blockmedian #1730
- [x] blockmode #1731
- [x] dimfilter #1956
- [ ] filter1d
- [x] nearneighbor #1919
- [ ] project
- [x] select #1756
- [x] sph2grd #1718
- [x] sphdistance #1916
- [x] sphinterpolate #1917
- [x] surface #1953
- [x] xyz2grd #1719
- [x] grd2xyz #1713
- [x] grdclip #1711
- [x] grdcut #1689
- [x] grdfill #1954
- [x] grdfilter
- [x] grdgradient #1720
- [x] grdlandmask #1721
- [x] grdproject #1722
- [x] grdsample #1724
- [x] grdtrack #1725
- [x] grdvolume #1726

**Are you willing to help implement and maintain this feature?** Yes