docs(hydro): add Quoting Code reference page and expand quoting gotchas into Quoting Limitations

[shadaj]

Adds a new user-facing "Quoting Code" page under the Stageleft reference section
(docs/docs/hydro/reference/stageleft/quoting.mdx) explaining:

• the two-stage execution model and what the q! macro does (graph construction
  vs. runtime, with a "what runs when" rule of thumb)
• capturing variables (free variables): snapshot semantics, the supported set of
  capturable types (integers, &str/String as &'static str), and workarounds for
  unsupported types
• Hydro's special free variables that resolve to live values at runtime:
  CLUSTER_SELF_ID and by_ref/by_mut state reference handles
• referencing module-level functions/types from quoted code

Rewrites the errors page as "Quoting Limitations", keeping the original three
gotchas and adding newly verified ones, each linked to a tracking issue on
hydro-project/stageleft:

• pub types must be used through their public API (private fields of pub structs
  fail at staged-compile time) — Quoted code can only use the public API of pub types (private fields fail at splice time) stageleft#96, related Handle lib_pub.rs generate for a pub struct inside a non-pub module stageleft#18/Handle pub structs inside #[cfg(test)] modules stageleft#43
• handwritten impl blocks on private types unavailable (derives work) — Handwritten impl blocks on private types are not available in quoted code stageleft#97
• capturing unsupported types (bool bool (and char, floats) cannot be captured as free variables in quoted code stageleft#99, Duration Add handling for Duration stageleft#55, general Create a python repr-esque trait for self-serialize/deserializing free variables stageleft#45)
• captures require move closures even for Copy types — Capturing free variables in quoted closures requires move, even for Copy types stageleft#98
• generic type parameters cannot be named inside q! — Support references to generic types in quoted code stageleft#47
• private declarative macros — Handling of private (non-#[macro_export]) declarative macros is broken stageleft#48
• helpers returning -> impl Trait — bug: -> impl Trait return types get partially expanded, breaking compilation stageleft#63
• multiple top-level statements crash — Crash in parsing stringified tokens when there are multiple top-level statements stageleft#20
• spurious unused_braces warnings in generated code — superfluous warnings about extra curly braces stageleft#91
• local functions need self:: (Local functions must be referenced with a self:: prefix inside q! stageleft#93), module-level imports (Function-local use imports are not visible inside q! stageleft#94), free variables
  in custom-syntax macros (Free variables are not detected inside macros with custom syntax stageleft#95)

Issues 93–99 were filed as part of this change; the behaviors were verified
empirically against a scratch copy of the stageleft test crates (private-field
access, private-type methods, derive behavior, capture errors, move requirement,
and generic-parameter splicing).

Also updates the Stageleft index page to link both pages and fixes the
quickstart link text. All compiled doc examples pass cargo test -p hydro_test --doc (mdtests), and the MDX was validated with @mdx-js/mdx.

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Pull request overview

This PR expands the Hydro Stageleft documentation by adding a new “Quoting Code” reference page and reworking the existing quoting errors page into a more comprehensive “Quoting Limitations” page, with links to Stageleft tracking issues for each limitation.

Changes:

• Added a new Stageleft reference page documenting the q! two-stage execution model, free-variable capture semantics, and special runtime-resolved handles.
• Rewrote the existing quoting errors page into “Quoting Limitations” with additional verified edge cases and links to Stageleft issues.
• Updated Stageleft index and Quickstart copy to link to and reference the new/renamed documentation.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
docs/docs/hydro/reference/stageleft/quoting.mdx	New “Quoting Code” page explaining q!, capture semantics, and special free variables.
docs/docs/hydro/reference/stageleft/index.mdx	Adds links to the new Quoting Code page and the Quoting Limitations page; refines note text.
docs/docs/hydro/reference/stageleft/errors.mdx	Retitles/restructures into “Quoting Limitations” and expands the catalog of documented gotchas with issue links.
docs/docs/hydro/learn/quickstart/index.mdx	Updates link text to refer to “Quoting Limitations” (same target file).

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