Add Exco bidder documentation

[saar120]

🏷 Type of documentation

• new bid adapter
• update bid adapter
• new feature
• text edit only (wording, typos)
• bugfix (code examples)
• new examples

📋 Checklist

• Related pull requests in prebid.js or server are linked -> Paste link in this list or reference it on the PR itself
• For new adapters check submitting your adapter docs

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[bretg]

I don't think the build failure is related to this PR. Please make the requested updates and the next run should succeed.

[muuki88]

@saar120 can you make the requested changes? You prebid adapter has been merged and now the documentation is eligible to merged as well.

[bretg]

Please fix the linting error

dev-docs/bidders/exco.md:37 MD031/blanks-around-fences Fenced code blocks should be surrounded by blank lines [Context: "```javascript"] 

[bretg]

Thanks for the docs fixes. This was part of PBJS 9.17. Merging the docs.