Improve custom input reader docs

[hombit]

1. It looks like an example at the "Catalog Import Arguments" is outdated and doesn't have the read_columns argument: https://hats-import.readthedocs.io/en/latest/catalogs/arguments.html#how-to-read-them
2. It also links this API docs page which doesn't have a lot of information: https://hats-import.readthedocs.io/en/latest/reference/api/hats_import.catalog.file_readers.InputReader.html#hats_import.catalog.file_readers.InputReader. Let's change it to https://hats-import.readthedocs.io/en/latest/reference/file_readers.html?
3. I propose to move the example to the "File Readers" page and maybe extend it by showing how to use read_columns to speed up the mapping stage.