Add `site_log_dir()` counterpart for `user_log_dir()`

[PhrozenByte]

The user_log_dir() method currently lacks a site_log_dir() counterpart.

Some suggestions:

• Unix: /var/log/$appname?
• Android: user_log_dir() (as with any other site_*_dir() counterpart)?
• Windows: site_data_dir() with an opinionated Log subdirectory?
• Mac OS: /Library/Logs/$appname?

This is a prerequisite for #213

[gaborbernat]

Triaging this — the proposed paths look correct based on platform conventions:

Platform	site_log_dir	Rationale
Unix	/var/log/$appname	Standard syslog convention
macOS	/Library/Logs/$appname	Mirrors user_log_dir (~/Library/Logs/$appname)
Windows	C:\ProgramData\$appauthor\$appname\Logs	site_data_dir + opinionated Logs (mirrors user_log_dir pattern)
Android	Same as user_log_dir	All site_* methods return user_* on Android

Implementation would touch: api.py (abstract property + path variant), all four platform modules, __init__.py (public functions), __main__.py (CLI), docs, and tests.

Working on a PR for this.

[gaborbernat]

Implemented in PR #423. Added site_log_dir, site_log_path, iter_log_dirs, and iter_log_paths across all platforms with the paths discussed above.