docs: simplify changelog entry format

[legendecas]

Which problem is this PR solving?

Refs: #2881 (comment)

Short description of the changes

Simplify the changelog entry format, so that it can be easier for people to manually fill in the entry.

Type of change

• This change requires a documentation update

Checklist:

• Followed the style guidelines of this project
• Documentation has been updated

[codecov]

Codecov Report

Merging #2889 (b0e685a) into main (9890767) will decrease coverage by 0.81%.
The diff coverage is n/a.

❗ Current head b0e685a differs from pull request most recent head 32e92e7. Consider uploading reports for the commit 32e92e7 to get more accurate results

@@            Coverage Diff             @@
##             main    #2889      +/-   ##
==========================================
- Coverage   93.42%   92.61%   -0.82%     
==========================================
  Files         162       66      -96     
  Lines        5564     1976    -3588     
  Branches     1175      432     -743     
==========================================
- Hits         5198     1830    -3368     
+ Misses        366      146     -220     

Impacted Files	Coverage Δ
...ckages/opentelemetry-sdk-trace-web/karma.worker.js	0.00% <0.00%> (-100.00%)	⬇️
...s/opentelemetry-core/src/platform/node/sdk-info.ts	0.00% <0.00%> (-100.00%)	⬇️
...opentelemetry-core/src/platform/node/globalThis.ts	0.00% <0.00%> (-100.00%)	⬇️
...pentelemetry-core/src/platform/node/performance.ts	0.00% <0.00%> (-100.00%)	⬇️
...emetry-api-metrics/src/platform/node/globalThis.ts	0.00% <0.00%> (-100.00%)	⬇️
...lemetry-resources/src/detectors/ProcessDetector.ts	31.81% <0.00%> (-68.19%)	⬇️
...ckages/opentelemetry-exporter-zipkin/src/zipkin.ts	84.48% <0.00%> (-15.52%)	⬇️
...lemetry-resources/src/detectors/BrowserDetector.ts	93.33% <0.00%> (-6.67%)	⬇️
...ckages/opentelemetry-sdk-metrics-base/src/Meter.ts
...sdk-metrics-base/src/exemplar/ExemplarReservoir.ts
... and 94 more

[dyladan]

Can you also update the guidance in https://github.com/open-telemetry/opentelemetry-js/blob/main/CONTRIBUTING.md#changelog

[pichlermarc]

Just realized that there's also the experimental/CHANGELOG.md file that needs to be updated. 🙂

[legendecas]

@pichlermarc updated experimental/CHANGELOG.md :D

[dyladan]

Seems fine. I'd merge it but the merge rules need another green check