arrow

GH-41410: [C++][FS][Azure][Docs] Add AzureFileSystem to Filesystems API reference (#41411)

### Rationale for this change

See https://github.com/apache/arrow/issues/41410.

### What changes are included in this PR?

Just changes to filesystem.rst.

### Are these changes tested?

Yes, locally.

### Are there any user-facing changes?

These are those changes.
* GitHub Issue: #41410

Authored-by: Bryce Mecum <petridish@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>