Merge pull request #25 from haiwen/windows-gc

add document for gc on windows
Add http sync documentation.
Merge pull request #26 from haiwen/http-sync

Add http sync documentation.
Improve docs on syncing via http/https
Add description for new architecture
Create file_permission_management.md
Update README.md
Update SUMMARY.md
Add clustering with MariaDB and Ceph
Update download_and_setup_seafile_professional_server.md
update cluster doc about office document preview
fixed typo
make cluster nodes more clear