Doc(eos_designs, eos_cli_config_gen): Various doc improvements

[carlbuchmann]

Change Summary

• Updates the landing page for GitHub and avd.arista.com
• Put all examples in new section "Examples" as opposed to "Getting Started".
• Augment details for custom templates with node type customization
• Add information for default_node_types
• Cut "Input Variables.md" from eos_cli_config_gen doc, no longer in use.
• Fix TODOs in porting guide
• Add tip to custom templates in eos_cli_config_gen
• Fix mermaid diagram rendering
  • Had to cut custom name for the classes, as MKDOCs do not support it
  • Investigated alternate MKDOCs plugins to change version, but none were found.
• Diagram for pyavd functions
• Fixes Fix(eos_designs): Doc typo in input variables #3019

Component(s) name

arista.avd.eos_designs

How to test

N/A

[carlbuchmann]

#3019 has been address with a2bbf1f