Reorganise PHP Mapscript documentation to get important information included in the TOC and PDF

[havatv]

This issue has been copied from mapserver/mapserver#4347.

I suggest that PHP Mapscript documentation is reorganised as follows:

• A new file is introduced: mapscript/php/phpmapscript.txt - This file will contain the API (currently in index.txt).
• mapscript/php/index.txt is changed to an index, referencing: phpmapscript.txt, by_example.txt, migration_guide.txt.
• mapscript/php/class_intro.txt is removed and its contents included in index.txt.

Currently, the migration guide and the by example file are not included in the documentation tree / table of contents, and they are not included in the PDF document. I think they should be!

I am willing to do this, but it would be nice to get input (and hopefully approval) from @jmckenna and @sdlime first.

[havatv]

OK. I did the restructuring for both 6.2 and master (they were identical before the changes). Hopefully it is OK.