docs(so): add Somali translation for how-to-contribute

[hamsehmahamud]

• Have you followed the contributing guidelines?
• Have you explained what your changes do, and why they add value to the Guides?

What this PR does:
This PR adds the Somali (so) translation for the "How to Contribute" guide.

Why it adds value:
It makes open-source contribution guidelines accessible to the Somali-speaking community, helping to expand the global reach and inclusivity of the Open Source Guides.

[hamsehmahamud]

"The tests are failing on the HTMLProofer check because it's looking for /so/. Since this is the first Somali translation, the Somali index/homepage doesn't exist yet. How would you like me to handle this? Should I translate the homepage in this PR as well?"

[hamsehmahamud]

"Hi team! I've added the Somali (so) translation for the 'How to Contribute' guide and resolved the markdown linting errors.

However, the CI is currently failing on the test suite (rake aborted! with 1 error) and the HTMLProofer is failing because it's looking for the /so/ homepage path. Since Somali is a brand-new language for the Open Source Guides, I believe the site configuration or locale data needs to be updated to initialize the language.

Could you point me in the right direction to configure the new language, or would a maintainer be able to initialize the /so/ route?"