docs: add link to provider implementation guide

Added a link to the "Adding a New Provider" documentation at the end of the
supported providers list on the homepage to help developers implement custom
provider backends.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: domenkozar

docs/src/content/docs/index.mdx

@@ -111,6 +111,8 @@ $ secretspec config init
 - [**1Password**](/providers/1password/) - Team-based password management
 - [**LastPass**](/providers/lastpass/) - Cloud-based password manager
 
+[Learn how to add a new provider →](/reference/adding-providers/)
+
 ```bash
 # Check all secrets are available and set them if not
 $ secretspec check