PIPE-2738: Update `hifa_diffgaincal` docstring with additional content and figures from User's Guide
- Migrated extended explanations and context from Section 9.21 of the ALMA Pipeline User's Guide to the `hifa_diffgaincal` CLI docstring.
- Added phase offset and residual weblog plots side-by-side (matching the PDF layout) and retained their original LaTeX filenames for traceability.
- Clarified the residual offset evaluation criteria and elaborated on the low-SNR heuristics logic.
- Fixed a Sphinx layout issue by adding a title to the `list-table` directive, preventing the `docstring_inheritance` parser from mistakenly treating the table as a new section header and mangling the `Args:` injection.
