chore(docs): Update link to kibana+fleet development in developers-guide.md

[biscout42]

current link points me to 404 🤷

[mergify]

This pull request does not have a backport label. Could you fix it @biscout42? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-./d./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[github-actions]

🔍 Preview links for changed docs

• docs/developers-guide.md

[biscout42]

Could you fix it

I'm sorry, seems I don't have permissions to add labels 🤔 . Can't edit