manual: Separate source file for each page
Split fc_manual.c so that each manual page has its own source file.
Have to split this, to have patches applying more cleanly - with manageable conflicts.
Settings -> #48742
Commands -> #48760
Terrain -> #48783
Improvements (both Buildings and Wonders): #48801 Governments: #48814 Units: #48824
Attached Tech page handling as the final patch for this series.
Split fc_manual.c so that each manual page has its own source file.