Render abstracts in <p> elements

Thanks @aphedges for your help in #6484. As of now we are hackily rendering paragraph breaks (``) in the XML as `

`. But ideally the abstract would be rendered in one or more `

` elements. Is this something you might be able to make a PR for?

No files or tests are named in the issue. Start by locating where abstracts are rendered from XML to HTML and search for `` handling. Trace that branch to the HTML output logic and update the mapping so paragraph breaks become `

` elements instead of `

`. Validate by building/checking a sample abstract page or any rendering test path and confirming expected `

` output.

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