add support for user-defined templates

[YouSysAdmin]

Hi :)

Changes:

• Adds support for user-defined templates for
  • Node (machine) register page
  • Apple/Windows client config help page
  • OIDC callback
• Adds doc about user-defined templates
• Moved all embedded templates in one place /hscontrol/html for comfortable managing
• For rendering templates using new pkg hscontrol/templates

Motivation:

• Multilanguage
• It's easy to change the design of these pages without maintaining a separate version of the app.
• Ability to add additional information for additional steps etc.

Summary by CodeRabbit

• New Features

  • Introduced custom templates for specific pages, enhancing user customisation options.
  • Added configuration option for user template directory, improving template management flexibility.
  • New HTML guides for configuring Tailscale on Apple devices and registering machines within the Headscale network.

• Bug Fixes

  • Streamlined OIDC callback handling for improved performance and maintainability.

• Documentation

  • Added comprehensive guide on using custom templates in the Headscale server environment.
  • Enhanced documentation structure with a dedicated link for custom templates.