[repo] Update docs with third-party code inclusion guidance

[CodeBlanch]

Relates to #5187

Changes

• Update CONTRIBUTING.md to have guidance for third-party code inclusion. Inspired by dotnet/runtime CONTRIBUTING.md.
• Add a note on PR template about license requirements.

Merge requirement checklist

• CONTRIBUTING guidelines followed (nullable enabled, static analysis, etc.)

[utpilla]

Would you be attributing the existing third-party code in the THIRD-PARTY-NOTICES.TXT in some follow-up PR?

[CodeBlanch]

That's what #5190 was all about. You think any more is needed?

[utpilla]

I was checking the dotnet runtime repo's THIRD-PARTY-NOTICES.TXT and their attributions are more specific where they list out the individual components/file copied along with the project URL that it's copied from.

I was wondering if we want to do that as well instead of having a blanket attribution to "Copyright (c) .NET Foundation and Contributors".

Just a thought. Not a blocker by any means.

[github-actions]

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[alanwest]

@CodeBlanch just a few links that need fixing

https://github.com/open-telemetry/opentelemetry-dotnet/actions/runs/7305743476/job/19909675451?pr=5195#step:4:834

[23-12-23 02:48:31.473]Warning:[BuildCore.Build Document.LinkPhaseHandlerWithIncremental.ConceptualDocumentProcessor.Save](CONTRIBUTING.md#L275)Invalid file link:(~/THIRD-PARTY-NOTICES.TXT).
[23-12-23 02:48:31.473]Warning:[BuildCore.Build Document.LinkPhaseHandlerWithIncremental.ConceptualDocumentProcessor.Save](CONTRIBUTING.md#L280)Invalid file link:(~/THIRD-PARTY-NOTICES.TXT).
[23-12-23 02:48:31.473]Warning:[BuildCore.Build Document.LinkPhaseHandlerWithIncremental.ConceptualDocumentProcessor.Save](CONTRIBUTING.md#L259)Invalid file link:(~/LICENSE.TXT).

[reyang]

@CodeBlanch FYI - adding *.TXT files to

opentelemetry-dotnet/docs/docfx.json

Lines 28 to 33 in d73ed67

	"resource": [
	{
	"files": [
	".editorconfig",
	"**.cs"
	]

should fix the docfx build failure.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0889e8d) 83.44% compared to head (0c5d3ff) 83.40%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5195      +/-   ##
==========================================
- Coverage   83.44%   83.40%   -0.04%     
==========================================
  Files         297      297              
  Lines       12531    12531              
==========================================
- Hits        10456    10451       -5     
- Misses       2075     2080       +5     

Flag	Coverage Δ
unittests	83.40% <ø> (-0.04%)	⬇️

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see 7 files with indirect coverage changes