Fix documentation scrolling and clarify site startup

[zcourts]

This change restores natural browser scrolling across Fission documentation sites. The generated documentation shell no longer traps the article inside a viewport-height scroll container or leaves the footer behaving like a second scroll stage. The browser document owns vertical scrolling, while the header remains sticky and the sidebar navigation remains independently usable when its contents exceed the viewport. The border-bearing sidebar columns stretch from the header to the footer, and the Fission website removes its old fixed page heights and excessive terminal whitespace. Previous and next documentation cards now target the actual inner navigation row, so long titles remain aligned on one row at desktop widths and collapse cleanly on mobile.

fission site serve now says that it is building before the local server starts, reports route rendering and writing progress throughout long static builds, and announces the transition to server startup. Serving … remains the readiness message emitted only after the listener has successfully bound its address. This makes Cargo’s earlier process-launch line much less likely to be mistaken for an accepting HTTP server.

The PR also adds a centralized environment-variable reference covering runtime shells, Web and native testing, renderer and diagnostic controls, storage, generated scripts, packaging, signing, and publishing. In particular, it documents that FISSION_WEB_TEST_CONTROL=1 must be present when the WASM application is compiled; setting it only while serving or running a browser test cannot add the test bridge to an existing build. The live-test guides and relevant crate READMEs link back to that authority.