Schema tightening

the relaxNG schema is our documentation for data sources which also checks validity.

Based on previous discussion, we should define valid values for months (see #94 and the second one where we hat days in the month field) and while at it, pages and doi seem to be low hanging fruits.

What is the regexp we want to accept for month?

Start by reading the RelaxNG schema and the prior discussions referenced in #94, including the discussion about days in the month field. Determine the accepted regular expressions for month, pages, and doi before making any change. Done means the schema documents and validates the agreed values without leaving the requirements unresolved.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.