Document product improvements

[tembo]

Summary

Added documentation for recent product improvements across billing, automations, and coding agents:

• Billing: Documented automatic default payment method setup during subscription upgrades
• Automations:
  • Expanded GitLab event triggers (MR updates, issue comments, pipeline failures, new refs)
  • Added Jira integration with advanced filtering capabilities
  • Enhanced Linear integration with label filtering
  • Added Atlassian MCP server for Jira/Confluence workflows
• Coding Agents:
  • Added claude-opus-4-6 model support
  • Expanded GPT-5.1 reasoning effort variants (minimal, low, medium, high)
  • Added gpt-oss model (OpenCode only)
  • Added Meta's llama-3.3-70b model (OpenCode only)
  • Updated Opencode agent description with expanded model support

Want tembo to make any changes? Add a review or comment with @tembo and i'll get back to work!

[tembo]

Requesting review from @ryw who has experience with the following files modified in this PR:

• admin/billing.mdx
• features/automations.mdx
• features/coding-agents.mdx