Tweaks to dbt and xAPI extending docs

[pomegranited]

Some minor modifications to the "Extending DBT" and "Extending xAPI" docs as a result of work on #222 :

• fixes some links and code references to match the actual plugin created
• links to the single most useful dbt debugging guide I found
• links to dbt model config docs

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[bmtcril]

Looks good, I think it just needs a rebase

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