Mention HeroDevs support in several areas.

[AndreAngelantoni]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the issue has not already been raised

Issue

[IN PROCESS]

Sister issue to fastify/fastify#5698.

Modify the following areas:

• Add Versions top-level menu item
• Modify banner on all version pages lower than 4.x (4.x will be done in the future when it reaches EOL)
• On v4 pages, add a different banner that provides a warning
• Update the Fastify README to include a version table with references to support for older versions.