Allow escaped special characters in the link text of the IEEEtran DOI note form #23

Currently, the IEEEtran DOI form of note added in #22 requires the link text to be identical to the DOI in the URL (regex backreference \{\1\} in check_url_field). For DOIs containing _, which are common for Springer chapters (e.g. 10.1007/11574620_45, 10.1007/978-3-642-25073-6_18), this forces a bare _ into the \href link text. The link text is typeset as normal text, so LaTeX fails with Missing $ inserted unless the document loads the underscore package. Escaping it as \_ fixes compilation, but the linter then reports the note as malformed. Other special characters (#, %, &, ~) have the same problem in principle.

The DOI in the link text should be compared to the URL after unescaping, so that e.g. \href{https://doi.org/10.1007/11574620_45}{10.1007/11574620\_45} is accepted. Additionally, docs/IEEEtran_best_practices.md should mention that a bare _ in the link text requires \usepackage[strings]{underscore}.