chore(template): Adjust template paths to be relative to site manifest directory

[ribose-jeffreylau]

Fixes #69

related: metanorma/metanorma-cli#375

NOTE: If the GHA jobs are failing due to:

/usr/local/bundle/gems/relaton-cli-1.20.0/lib/relaton/cli/xml_to_html_renderer.rb:42:in `read':
No such file or directory @ rb_sysopen - _relaton_templates/_index.liquid (Errno::ENOENT)

... then that means a new metanorma-cli release has not been published yet.

In that case, wait until a new release is done, then re-run the jobs, and merge as soon as they pass.