Add how-to guide for implementing and using custom crossmatch algorithms

Add a user-facing how-to guide that shows astronomers/developers how to define, validate, and run a custom crossmatch algorithm in lsdb by subclassing `AbstractCrossmatchAlgorithm`.

**Context**

- Why is this needed: Custom algorithms are supported but currently only discoverable mostly via docstrings/tests
- Success metric: Users can implement a minimal custom algorithm from docs without reading internals; reduced support questions around custom algorithm integration.

**Goals**

- New how-to guide added under tutorials/how-to docs explaining custom algorithm support.
- Guide includes a description of creating custom class subclassing [AbstractCrossmatchAlgorithm]
- Example execution with [Catalog.crossmatch(..., algorithm=MyAlgorithm(...))](vscode-file://vscode-app/Applications/Visual%20Studio%20Code.app/Contents/Resources/app/out/vs/code/electron-browser/workbench/workbench.html)
- Show difference from the results of the execution with KDTree algorithm
- Guide links to relevant API references.