docs: improve the contributing guide for doc writers

[tbouffard]

• disabling attribute substitution: improve syntax (the "+" character wasn't correctly interpreted)
• convert markdown to AsciiDoc listing
• improve wording
• add how-to define "internal references"

Notes

Based on fixes done in https://github.com/bonitasoft/bonita-doc/pull/2425

[sonarcloud[bot]]

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