Add more cross-references to the chunk encoding section

[maxrjones]

I usually feel deep in a rabbit hole when looking for the definition of a codec in the Zarr spec (I'm usually looking for https://zarr-specs.readthedocs.io/en/latest/v3/core/index.html#chunk-encoding and the definition that a "codec specifies a bidirectional transform (an encode transform and a decode transform))". While I know bookmarking this section would be a smart individual solution at this point, I expect others may also struggle to navigate this part of the spec and am therefore raising an issue. I think it could be useful to link to this section in a few other spots that are more prominent in the spec doc:

1. Link to https://zarr-specs.readthedocs.io/en/latest/v3/core/index.html#chunk-encoding from https://zarr-specs.readthedocs.io/en/latest/v3/codecs/index.html
2. Link to https://zarr-specs.readthedocs.io/en/latest/v3/core/index.html#chunk-encoding from https://github.com/zarr-developers/zarr-extensions/tree/main/codecs (which can be found from location 1)
3. Link to https://zarr-specs.readthedocs.io/en/latest/v3/core/index.html#chunk-encoding from https://zarr-specs.readthedocs.io/en/latest/v3/core/index.html#array-metadata-codecs

[joshmoore]

👍 see also #357