[#1131] Escape the option placeholders of the generated reference, so AsciiDoc no longer reads them as missing attributes

[vharseko]

Fixes #1131

The generated reference wrote option value placeholders ({name}, {unit}, {property}, …) as they are in the messages, and AsciiDoc read each of them as a reference to a missing attribute — 3,521 occurrences over the 220 generated man pages, the ~3,500 skipping reference to missing attribute warnings of the site build.

• Every template of the generated reference — those ArgumentParser/SubCommandArgumentParser apply and those DSConfig applies — goes through DocGenerationHelper.applyTemplate, which now escapes each attribute reference of the result with a backslash (\{name}). The .ftl templates, GenerateRefEntriesMojo and AsciidocConverterUtils are unchanged.
• A reference already escaped is left as is: the templates nest (a subcommand section inside the subcommands section inside the page), and escaping each level again would give \\{name}.
• Word characters are Unicode ones, as in Asciidoctor: the French bundle has {données}.
• {PROP:VALUE}, {name=value} and the like are no attribute references in AsciiDoc and stay as they are.
• :attribute-missing: skip in each partial was not used: set in an included partial, it would change the setting for the rest of the including page as well.

Verification

• DocGenerationHelperTestCase: a tool without and a tool with subcommands, through getUsage() in gendoc mode; red before the fix, and red again with the "already escaped" lookbehind removed.
• Generated all 34 tools listed in opendj-doc-generated-ref/pom.xml (220 pages, dsconfig split) the way GenerateRefEntriesMojo does, before and after: 3,521 unescaped references → 0, each escaped exactly once; unescaping the result gives back the original pages byte for byte.
• Rendered all 220 pages with AsciidoctorJ 2.5.3, html5 and manpage backends, -a attribute-missing=warn as Antora sets it: 6,451 missing-attribute warnings → 0; the HTML and man output is byte for byte the same as before, {name} still shown as is.
• opendj-cli (48) and opendj-core (8,182) tests pass; javadoc builds.
• The full Antora site build was not run locally.

Not in this PR: 920 of the {unit} occurrences come from another defect — in the dsconfig list-*/get-* subcommands, every option that handles no property (--unit-size, --unit-time, --property) gets the whole subtype list "… depends on the {unit} you provide", not only the name option.

[maximthomas]

praise: The escape sits at the one place every generated reference page passes through.

• All 12 template calls (ArgumentParser ×2, SubCommandArgumentParser ×6, DSConfig ×4) go through DocGenerationHelper.applyTemplate, so the single change at DocGenerationHelper.java:83 covers the dsconfig pages too.
• The (?<!\\) lookbehind keeps nested templates from escaping twice, and subcommandReferenceEscapesPlaceholdersOnce pins it with doesNotContain("\\\\{").
• {PROP:VALUE} and {name=value} are asserted unchanged in both cases.