Run xmllint validation only once

Before, each documentation target that built something from
postgres.sgml ran xmllint first to validate the input.  Here, we
change it so that the validation only runs once and produces an output
file, and all the other targets build from that output file.  This
avoids redundant work when building multiple documentation targets
(such as html and man).

Also, when we run xmllint, we can resolve entities (included files).
This helps with tools that don't support vpath builds, such as
dbtoepub.

All this also organizes the make targets a bit better for implementing
equivalent steps in meson.

Discussion: https://www.postgresql.org/message-id/e3ae16de-c9f9-f559-2d11-70b1342ae3d1@enterprisedb.com

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