That's a pretty major set of requirements you've been tasked with in your first year on the job. I don't have a good set of links for suggestions on this. I'd suggest checking out Simple-Talk to see what's there. [Here's a list of articles] on documenting ETL. There may be a couple there that are useful. Here's one on [testing documentation]. It looks pretty sparse. There are tons and tons of documents on [deployment methodologies and documentation] over there, including several that I've written.
The best way to think about documentation is that you need to write down enough information so everyone on the team knows what they need to do. But, don't write so much down that you're actually doing peoples jobs for them. That's serious over-documentation. The best thing you can do, spit out multiple drafts. Never let anything be finished and tell everyone that. Also, put the documents in a central location that can be accessed by the team. When someone says, "Hey, your document doesn't say anything about X" you can respond "Wow, you're right. Can you do a first draft on that since you're the expert?" Then, the processes start to document themselves.
: https://www.red-gate.com/Search/?t=simpletalk&s=document+etl : https://www.simple-talk.com/search/default.aspx?search=document+testing&x=0&y=0 : https://www.simple-talk.com/search/default.aspx?search=document+deployment&x=0&y=0
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