Clarify Caddyfile placeholder timing

[steadytao]

Summary

• clarify that Caddyfile placeholders are usually evaluated at runtime by supporting modules
• distinguish runtime placeholders from parse-time Caddyfile mechanisms such as import and {$ENV}
• note that fields compiled during config load, such as regexp matcher patterns may not support runtime placeholders as input values

Should close caddyserver/caddy#7721

[francislavoie]

Should link to import/snippets section and env section with those mentions

[steadytao]

Should link to import/snippets section and env section with those mentions

Linked them more explicitly.