Add docs for `for_particle_neighbor`

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 96.77419% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.19%. Comparing base (591827b) to head (fa11fc9). Report is 6 commits behind head on main.

Files	Patch %	Lines
src/neighborhood_search.jl	89.47%	2 Missing :warning:
src/nhs_grid.jl	96.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #23 +/- ## ========================================== + Coverage 88.07% 90.19% +2.11% ========================================== Files 11 11 Lines 260 255 -5 ========================================== + Hits 229 230 +1 + Misses 31 25 -6 ``` | [Flag](https://app.codecov.io/gh/trixi-framework/PointNeighbors.jl/pull/23/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | Coverage Δ | | |---|---|---| | [unit](https://app.codecov.io/gh/trixi-framework/PointNeighbors.jl/pull/23/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework) | `90.19% <96.77%> (+2.11%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trixi-framework#carryforward-flags-in-the-pull-request-comment) to find out more.

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