Update job writing guide docs

[mtuchi]

Short Description

Improve job writing guide docs by grouping into smaller topics and add warning message on Referencing credentials in job code section. [See advanced concept page]

Closes #714

Details

• Group job writing guide content into specific topic files and update sidebar
• Add a warning about Referencing credential secretes in job code

AI Usage

Please disclose how you've used AI in this work (it's cool, we just want to
know!):

• Code generation (copilot but not intellisense)
• Learning or fact checking
• Strategy / design
• Optimisation / refactoring
• Translation / spellchecking / doc gen
• Other
• I have not used AI

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Responsible AI Policy

[mtuchi]

Hey @hunterachieng Please review my pr and share your feedback, See my quick walk through video here