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
• [x] Strategy / design
• [ ] Optimisation / refactoring
• [ ] Translation / spellchecking / doc gen
• [ ] Other
• [ ] I have not used AI

You can read more details in our Responsible AI Policy

[mtuchi]

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