docs: drop the truncated-Available note on gpu.md (CTBase#557 fixed)

[ocots]

Small follow-up to #941. That PR shipped gpu.md's AmbiguousDescription diagnostic with a note explaining Available was truncated at 10 entries with no marker, silently dropping the two :gpu methods it exists to show — a bug filed as CTBase#553.

CTBase's own fix (0.30.3-beta) turned out to swap the problem rather than solve it: Available started showing 5 of 12 methods (the closest matches) with nothing saying so, and the Hint line ended on a colon with nothing after it. Filed as CTBase#557, fixed in CTBase#558, released as CTBase 0.30.4-beta — both same day.

This PR bumps the docs environment to 0.30.4-beta and drops the note. Verified on the actual rebuilt page, not just the fix in isolation — solve(ocp, :adnlp, :gpu) now renders:

julia> solve(ocp, :adnlp, :gpu)
AmbiguousDescription → _complete_description, strategy_builders.jl:260
│
│  Diagnostic  No complete match — no description contains all symbols
│  Requested   (:adnlp, :gpu)
│  Available   (:collocation, :adnlp, :ipopt, :cpu)
│              … all 12, GPU entries included …
│              (:collocation, :exa, :madnlp, :gpu)
│              (:collocation, :exa, :madncl, :gpu)
│
│  Context     description completion
│  Hint        Try one of the closest matches:
└─

docs/Project.toml's CTBase = "0.30" already covers 0.30.4-beta, so no [compat] change — only the tracked docs/src/assets/Manifest.toml snapshot, regenerated by the build.

Verification

Full local build (julia --project=. docs/make.jl), exit 0: 0 Cannot resolve @ref, 6 @extref — the unchanged pre-existing backlog. No new .png.

🤖 Generated with Claude Code