GitLab

* files.texi (Format Conversion): Expand intro; add menu.
(Format Conversion Overview, Format Conversion Round-Trip)
(Format Conversion Piecemeal): New nodes/subsections.
* hooks.texi: Xref "Format Conversion" , not "Saving Properties".
* text.texi (Text Properties): Remove "Saving Properties" from menu.
(Saving Properties): Delete node/subsection.