run_locally() add root_dir: str | Path | None keyword

[janosh]

So users don't have to change directory to control where flows run.

Added a test and docs.

[codecov]

Codecov Report

Merging #486 (bf36d98) into main (61bbaa4) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head bf36d98 differs from pull request most recent head a8d4587. Consider uploading reports for the commit a8d4587 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #486   +/-   ##
=======================================
  Coverage   99.86%   99.86%           
=======================================
  Files          20       20           
  Lines        1508     1509    +1     
  Branches      415      415           
=======================================
+ Hits         1506     1507    +1     
  Misses          2        2           

Files	Coverage Δ
src/jobflow/managers/local.py	100.00% <100.00%> (ø)