docs: fix pre-existing broken xrefs in index, quick-start, and 9k

Eight xrefs in `index.adoc` and `quick-start.adoc` used the old flat
path style (e.g. `coroutines/tasks.adoc`) instead of the numbered prefix
the rest of the site uses (`4.coroutines/4a.tasks.adoc`). Antora was
emitting "target of xref not found" errors on every build. Updated all
eight to point at the actual page paths.

The ninth error was in `9k.Executor.adoc` (formerly `8k.Executor.adoc`),
which linked to `../continuation-rationale.adoc` — that file is a
markdown design note (`doc/continuation-rationale.md`), not an Antora
page, so the xref could never resolve. Removed the broken link; the
surrounding prose still describes what `continuation` is.

Antora build now reports zero errors.

Author: sgerbino

doc/modules/ROOT/pages/9.design/9k.Executor.adoc

@@ -43,7 +43,7 @@ If the executor determines it is safe (e.g., the current thread is already assoc
 
 Queues the continuation for later execution without ever executing it inline. Never blocks. The continuation is linked into the executor's internal queue via its intrusive `next` pointer -- no per-post heap allocation.
 
-Both operations accept `continuation&` rather than `std::coroutine_handle<>`. A `continuation` wraps a coroutine handle with an intrusive list pointer, enabling zero-allocation queuing. See xref:../continuation-rationale.adoc[Continuation Rationale] for the design of this type.
+Both operations accept `continuation&` rather than `std::coroutine_handle<>`. A `continuation` wraps a coroutine handle with an intrusive list pointer, enabling zero-allocation queuing.
 
 The remaining operations support context access, lifecycle management, and identity:
 

doc/modules/ROOT/pages/index.adoc

@@ -114,8 +114,8 @@ The `task<>` return type (equivalent to `task<void>`) creates a lazy coroutine t
 == Next Steps
 
 * xref:quick-start.adoc[Quick Start] — Set up your first Capy project
-* xref:cpp20-coroutines/foundations.adoc[{cpp}20 Coroutines Tutorial] — Learn coroutines from the ground up
-* xref:concurrency/foundations.adoc[Concurrency Tutorial] — Understand threads, mutexes, and synchronization
-* xref:coroutines/tasks.adoc[Coroutines in Capy] — Deep dive into `task<T>` and the IoAwaitable protocol
-* xref:buffers/overview.adoc[Buffer Sequences] — Master the concept-driven buffer model
-* xref:streams/overview.adoc[Stream Concepts] — Understand the six stream concepts
+* xref:2.cpp20-coroutines/2a.foundations.adoc[{cpp}20 Coroutines Tutorial] — Learn coroutines from the ground up
+* xref:3.concurrency/3a.foundations.adoc[Concurrency Tutorial] — Understand threads, mutexes, and synchronization
+* xref:4.coroutines/4a.tasks.adoc[Coroutines in Capy] — Deep dive into `task<T>` and the IoAwaitable protocol
+* xref:5.buffers/5a.overview.adoc[Buffer Sequences] — Master the concept-driven buffer model
+* xref:6.streams/6a.overview.adoc[Stream Concepts] — Understand the six stream concepts

doc/modules/ROOT/pages/quick-start.adoc

@@ -115,6 +115,6 @@ capy::run_async(executor)(might_fail(),
 
 Now that you have a working program:
 
-* xref:coroutines/tasks.adoc[Tasks] — Learn how lazy tasks work
-* xref:coroutines/launching.adoc[Launching Tasks] — Understand `run_async` in detail
-* xref:coroutines/affinity.adoc[Executor Affinity] — Control where coroutines execute
+* xref:4.coroutines/4a.tasks.adoc[Tasks] — Learn how lazy tasks work
+* xref:4.coroutines/4b.launching.adoc[Launching Tasks] — Understand `run_async` in detail
+* xref:4.coroutines/4c.executors.adoc[Executors and Execution Contexts] — Control where coroutines execute