Broken links to internal style guide

[MV1205]

Describe the bug

Broken links to the nginx/documentation style guide:

• A broken link to the style guide was found on the documentation/CONTRIBUTING.md page

• A broken link to the style guide was found on Issue #228

To reproduce

Steps to reproduce the behavior:

1. Go to either documentation/CONTRIBUTING.md page or Issue Updates to product titles based on style updates #228 page
2. Click on the style guide link.
3. See 404 page not found error.

Expected behavior

After clicking the link to the style guide, expect to be directed to documentation/style-guide.md.