docs: add mappers docs guide

[pnadolny13]

Related to #6412 (comment)

Adds a guide to the docs that includes both standalone mappers and SDK based inline stream maps

• I sourced a lot of this information from https://sdk.meltano.com/en/latest/stream_maps.html and some examples from docs: #6412 add examples for usage MeltanoLabs/meltano-map-transform#117
• I put a couple examples to give a preview of what they do but I mostly wanted to link out to other docs like docs: #6412 add examples for usage MeltanoLabs/meltano-map-transform#117 and https://sdk.meltano.com/en/latest/stream_maps.html
• I felt like it was worth highlighting the difference between the two mapping methods and when you would use each, with tradeoffs. So let me know if I got anything wrong.
• Its a bit tricky to figure out the best way to refer to the two different approach. I went with "standalone mapper plugins" and "SDK

Let me know if you have any obvious use cases I missed and what you think in general.

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[tayloramurphy]

@pnadolny13 minor comments, but this looks great. Coupled with the PR on the mapper repo, I'm quite happy with these updates.