Provide a conventions section

Detail conventions used in all books. First and only conventions detailed
for now: prompts and admonitions.

Use a common ch_preface.xml for all books.

Change-Id: I7ed5c8b63ed9277dfdbccc55eb76e97e364118c8
change to section_getstart_compute.xml

SQLite3 was changed - spell

Change-Id: I0332c3b77a3ce4d21e0482b0a0fe3b89cdfdb0a2
Merge "Provide a conventions section"
Merge "change to section_getstart_compute.xml"