Documentation for bootstrap data conversion

I felt it was worth spending some extra effort on documentation for
this change, since it's going to impact a lot of future patches.
Accordingly, I've taken John's proposed README text and moved it
into the SGML format, and done a fair amount of editing to extend
the text and bring it all up to date.

The attached patch only touches bki.sgml, but I intend that
src/backend/catalog/README would go away altogether, and we'd
likewise get rid of the separate README.data file that John proposed.
Pretty much all the useful info in those files is here in some form.

One question is whether it's worth renaming bki.sgml to something
more in keeping with its new focus. I'm inclined not to, because
presumably that would also entail changing the chapter ID to something
else, and I'm not sure how well our documentation website would cope
with that.

John and I are probably both too close to the patch to be able to
review this documentation for clarity and usefulness, so if anyone
else wants to have a look, please comment.

regards, tom lane