fix: warning if dagu is using legacy directory structure

[arky]

Show a warning if legacy configuration path are being used. Also add note about this in the configuration section of the docs.

Original feedback from Discord: https://discord.com/channels/1095289480774172772/1095291629344149544/1340679589185327114

We should have a section on backward compatibility where the $HOME/.dagu kind of directory structure is also supported.
Or we should have a clean migration guide mentioning from which version we started supporting new directory structure to avoid confusion at user end

We can provide user an command line option in dagu command which can help moving from old directory structure to new directory structure. Or when starting dagu, it can provide user message that older directory structure detected, would you like to migrate to new directory structure.

We can have an informative message while starting dagu mentioning older directory structure detected (supported for backward compatibility). For upgrading to latest structure, please visit: https://dagu.readthedocs.io/en/latest/config.html

[arky]

@yohamta is working on this fix.