Fix 'mkdocs serve' bug - issue #45

[mchaaler]

Fix 'mkdocs serve' bug - replace startswith by conversion to pathlib object + relative_to() method

Fixes #45

[smarie]

Nice ! thanks @mchaaler. I have doubts that this actually does the entire job (it fixes the error but may trigger additional useless builds)

[mchaaler]

Manual test is OK:
✅ run mkdocs serve
✅ modify a file in the source dir: it should update the site
✅ modify a file in the doc/generated/gallery files: it should not trigger a site recreate, since the path is in excluded_dirs

[smarie]

Thanks so much for your efforts and patience @mchaaler !
Times have been complicated for me lately in terms of bandwidth for such project. I made a minor suggestion, moving from ValueError to TypeError when the type of paths objects in not Path. I'll merge when the build is complete

[smarie]

done! thanks again