Eliminate PDF doc build margin problem [skip appveyor]

This is an AI analysis of errors reported by Apache fop.  The error
looks like this (same for margin-right - line-breaks mine):

[ERROR] FOUserAgent - Invalid property value encountered in margin-left="":
org.apache.fop.fo.expr.PropertyException:
file:/home/mats/github/scons/build/doc/user/scons-user.fo:3:6:
No conversion defined ; property:'margin-left' (See position 147:8)

Some builds of fop report the error as SEVERE rather than ERROR.

The problem was identified as:

A. When `$fop.extensions != 0`, the `<xsl:value-of>` inside
   `<xsl:attribute>` elements produces whitespace-padded content that
   FOP can't parse as valid length values

B. When `$fop.extensions = 0`, the `<xsl:attribute>` elements with empty
   content produce empty strings

The solution is to use `<xsl:choose>` blocks that always produce
valid length values:
- When `$fop.extensions != 0`: use `0pt` (simple, valid FOP length)
- When `$fop.extensions = 0`: use the original dynamic variable values

To summarize the two-part fix across all four `scons_title.xsl` files:

1. (1 pair per file): Removed the redundant `xsl:attribute` elements
   from `scons-titlepage-first` that conflicted with the literal
   `margin-left="0mm" margin-right="0mm"`

2. (11 blocks per file, already applied): Replaced the broken
   content-mode `xsl:attribute` blocks that produced empty strings or
   malformed `concat()` values:

Note: we only build doc/man and doc/user, but the same fixes were
applied to the two unused documents for consistency.

Signed-off-by: Mats Wichmann <mats@linux.com>
Assisted-by: OpenCode 1.18.5 driving Qwen 3.8

Author: mwichmann

CHANGES.txt

@@ -149,6 +149,7 @@ RELEASE  VERSION/DATE TO BE FILLED IN LATER
     - Clarify internal usage of zipimporter (not user-visible code)
     - Fixed an old problem in PDF doc generation where the path to an
       image file ended up with a "not found" error.
+    - Fixed a PDF doc build problem relating to illegal margin widths ("").
     - Reference manual improvements:
       * More clarifications in Builder Methods section.
       * Clarify VariantDir behavior when switching from duplicate=True (the

RELEASE.txt

@@ -187,6 +187,8 @@ DOCUMENTATION
 - Fixed an old problem in PDF doc generation where the path to an
   image file ended up with a "not found" error.
 
+- Fixed a PDF doc build problem relating to illegal margin widths ("").
+
 DEVELOPMENT
 -----------