DOC: Add a release note for PDF font embedding fixes

Author: QuLogic

doc/release/next_whats_new/pdf_fonts.rst

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+Improved font embedding in PDF
+------------------------------
+
+Both Type 3 and Type 42 fonts (see `fonts`_ for more details) are now embedded
+into PDFs without limitation. Embedded fonts may be split into multiple subsets
+in order to satisfy format limits. Additionally, a corrected Unicode mapping is
+added for each.
+
+This means that _all_ text should now be selectable and copyable in PDF viewers
+that support doing so.