docs: add explicit project links to contributing index (#7278)

[LoveChauhan-18]

• I have read and followed the Contributing docs, especially the "First-time contributing?" section.
• This PR has content that I did not fully write myself.
  • I used AI and I have read and followed the Generative AI Contribution Policy.
• I have the experience and knowledge necessary to understand, review, and validate all content in this PR.¹

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Fixes #7278

This PR updates the "What can I contribute to?" section in the contributing _index.md to directly list the main OpenTelemetry repositories (Specification, Semantic Conventions, Collector, and Language SDKs).

Previously, users had to dig through the general community repo or specs to find where to contribute (like the semconv docs). This change surfaces the specific repository links right in the navigation, making them much easier to discover for new contributors.

Footnotes

1. Yes, I can answer maintainer questions about the content of this PR, without using AI. ↩

[vitorvasc]

I feel we can discuss this further before merging, as per @svrnm's comment. Personally, I also like the idea of keeping this section clean and focused on documentation contributions.

Instead, we could expand the contribution possibilities by creating a new page/section (maybe /community/contribute?) that includes this list, the overall guidelines, and other content that is currently available only through the community repo. WDYT?

/cc @open-telemetry/docs-approvers

[maryliag]

I don't think this should be added here. There are even more repos that were mentioned here, so this could make it sound the missing ones are not "worth it", which is not true. Also, new repos are being created every now and then and keeping this up to date could be challenging. One option would be to link to the github page that list all otel repos, keeping it generic, and mentioning each one has a tab for "contributing" and more specific details can be found on them