docs: Document usage of new go tools feature in go1.24.0

[joerdav]

With the release of go1.24.0, there is a new feature allowing tools to be tracked in the go.mod.

You can run go get -tool github.com/a-h/templ/cmd/templ@latest to add the latest templ version as a tool.

Then you can run templ as normal using go tool templ. For example go tool templ generate.

This links the generator version to the runtime version specified in the go.mod. So you reduce the likelihood of generated code that is incompatible with the runtime version.

There should be a docs section outlining this, or at least that links to the go docs as a recommended way of tracking templ versions.