Small fixes to Docker Install guide

[lasomethingsomething]

A few improvements to flow:

• Bottom of https://developer.shopware.com/docs/guides/installation/ states, "Next, check the hardware requirements to prepare your system for installation." But the "Next page" at bottom right goes to "Legacy Setups." Let's make it go to "System Requirements."
• add an EOL to address tags issue (some don't get updates anymore); we don't have this right now. PHP versions go EOL, then we have that. We need versioning for the Docker image.
• Install guide puts "Legacy Setups" above the Docker setup, sending a mixed message. (See image). Let's put Legacy Setups at the end of the section.