Add special docs.rs feature to enable all-is-cubes/std.

Without this, `all_is_cubes_mesh::dynamic::MeshJobQueue` is documented
as `!Send` when it ought to be conditionally `Send`.

The crates that need this extra feature are crates that don't already
enable all-is-cubes/std through any feature at all. all-is-cubes-mesh
Is the only one that actually meets that condition, but
all-is-cubes-content only depends indirectly in a way that I want to
remove, so I added the feature there too.

Also added an empty metadata section to all-is-cubes-ui, to remind
myself to fill it out as needed in the future.

all-is-cubes-content/Cargo.toml

@@ -12,7 +12,7 @@ categories = ["games"]
 keywords = ["all-is-cubes"]
 
 [package.metadata.docs.rs]
-features = ["arbitrary"]
+features = ["_full-documentation"]
 
 [lib]
 # Disable running as benchmark so that the default doesn't interfere with Criterion usage.
@@ -25,6 +25,9 @@ harness = false
 [features]
 # Adds `UniverseTemplate::Random`.
 arbitrary = ["dep:arbitrary", "all-is-cubes/arbitrary"]
+# Internal use only. Used to ensure that docs.rs builds have complete information;
+# in particular, `all-is-cubes/std` would otherwise be missing, making types appear `!Sync`.
+_full-documentation = ["all-is-cubes/std", "arbitrary"]
 
 [dependencies]
 all-is-cubes = { workspace = true }

all-is-cubes-mesh/Cargo.toml

@@ -12,7 +12,7 @@ categories = ["games", "graphics", "rendering"]
 keywords = ["all-is-cubes", "voxel", "mesh"]
 
 [package.metadata.docs.rs]
-features = []
+features = ["_full-documentation"]
 
 [lib]
 # Disable running as benchmark so that the default doesn't interfere with Criterion usage.
@@ -39,6 +39,9 @@ arbitrary = ["dep:arbitrary", "all-is-cubes/arbitrary", "ordered-float/arbitrary
 # If disabled, `std::thread` is never used.
 # If enabled, threads may be created and the active `rayon` thread pool may be used.
 auto-threads = ["dep:rayon"]
+# Internal use only. Used to ensure that docs.rs builds have complete information;
+# in particular, `all-is-cubes/std` would otherwise be missing, making types appear `!Sync`.
+_full-documentation = ["all-is-cubes/std", "arbitrary"]
 
 
 [dependencies]

all-is-cubes-ui/Cargo.toml

@@ -11,6 +11,9 @@ license = "MIT OR Apache-2.0"
 categories = ["games", "gui"]
 keywords = ["all-is-cubes", "voxel"]
 
+[package.metadata.docs.rs]
+features = []
+
 [lib]
 # Disable running as benchmark so that the default doesn't interfere with Criterion usage.
 bench = false