Exclude buf.yaml from release archives via .gitattributes (#604)

[RajaVarmaGVSSR]

Adds an export-ignore rule for buf.yaml to prevent it from appearing in GitHub release
tarballs and zip files.

Background:

• Release archives currently include all tracked files, which previously caused
  buf.yaml to appear in the tarball.
• package.json and package-lock.json are no longer in the repo, so no action needed.
• Existing release workflow remains unchanged.

Closes #604

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: lmolkova / name: Liudmila Molkova (16b33f0)

[RajaVarmaGVSSR]

Please consider adding hacktoberfest-accepted label so this PR can count for Hacktoberfest. Thanks!

[RajaVarmaGVSSR]

@tigrannajaryan, sorry I am new to this. Are there specific reviewers or groups I need to ask for reviews ?

[tigrannajaryan]

@open-telemetry/specs-sponsors @open-telemetry/technical-committee please review.

[lmolkova]

Is there actually a need to remove this file? Does it create any problems?

It's part of the validation and code-generation and we ship proto files as they are in the archive. We expect consumers of the release to generate code. In a generic case, without this file, consumers might not be able to generate code from the proto we provide.