docs: Document the SPA and static cache control environment variables

[developerjamiu]

Companion to #756. The web server ships three cache-control environment variables built on the same mechanism; #756 documents the FlutterRoute one, and this adds the other two on their own pages:

• SERVERPOD_WEB_SERVER_STATIC_CACHE_CONTROL on the Static files page, next to the cache-control helpers it substitutes for.
• SERVERPOD_WEB_SERVER_SPA_CACHE_CONTROL on the Single-page apps page, including the two SpaRoute-specific rules: the fallback file is always served without a Cache-Control header, and the static variable does not apply to SpaRoute.

In all three cases an explicit cacheControlFactory takes precedence over the environment variable.

[SandPod]

Question - Have we listed these on the ENV page? What is the strategy around that currently?

[developerjamiu]

Not yet (thanks for the catch). The reference only listed variables that also have a config-file option, and these three are env-only. I'll add all three (including #756's FlutterRoute one) to the table with links to their pages. Going forward the rule is: every SERVERPOD_* variable gets a row in the reference, and the full explanation lives on the feature's own page.