docs : update HOWTO-add-model.md (#23883)
* docs: update HOWTO-add-model.md with new model registration and graph-building instructions
* docs: improve formatting in HOWTO-add-model.md
* Update docs/development/HOWTO-add-model.md
Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>
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Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>
