Improve developer README

[gwenwindflower]

This is a:

• [x] documentation update
• [ ] bug fix with no breaking changes
• [ ] new functionality
• [ ] a breaking change

Description & motivation

I found, even as an experienced analytics engineer, being new to package development that the developer README for contributors in integration_tests/README.md could use some expansion and clarifying to be more welcoming. I've attempted to do this.

In doing this I also wanted to clarify the .env / profiles.yml flow to be simpler, as that was my first initial source of confusion. This PR proposes that we:

• store the CI profiles.yml in the root of the project so it's more discoverable and saves us a step in CI
• get rid of the .env/[adapter].env files, which are run the risk of somebody filling in and committing their credentials, and instead list the env vars needed for each adapter in the expanded README.

Checklist

• [n/a] This code is associated with an issue which has been triaged and accepted for development.
• [x] I have verified that these changes work locally
• [x] I have updated the README.md (if applicable)
• [n/a] I have added tests & descriptions to my models (and macros if applicable)
• [ ] I have added an entry to CHANGELOG.md

[gwenwindflower]

Approved at a vibes level as opposed to a line-by-line review of the code! I like your plan and justification, and believe in your implementation

ty 💗 -- i really wish the markdown had already been auto-linted/formatted as it makes it hard to see exactly what the intentional changes were, as opposed to all the formatting that was automatically applied by Prettier updating the markdown, but now it will be clean moving forward (everybody using VSCode will have the same auto-formatting/linting of the Markdown as this, so should be pretty universal).

i'll add some details re make installation and get this in. 🙏🏻