Fix spelling, grammar, and stale identifiers in docs and Javadocs

- doc/reference/concurrency-1.md: fix misspellings (exmaple_thread,
  initiazer), grammar errors, and remove duplicated content
- doc 4c.executors.adoc: correct "two methods" to "three methods"
- doc 8l.RunApi.adoc: fix garbled section symbol
- doc why-capy.adoc: hyphenate "twenty-five", fix "with over decades"
- task.hpp, thread_pool.hpp: fix stale repository URLs
- io_awaitable.hpp, io_awaitable_promise_base.hpp: env->allocator and
  this_coro::allocator to env->frame_allocator and
  this_coro::frame_allocator
- dynamic_buffer.hpp: string_buffers to string_dynamic_buffer with
  correct pointer syntax in code examples

Author: sgerbino

doc/modules/ROOT/pages/4.coroutines/4c.executors.adoc

@@ -9,7 +9,7 @@ This section explains executors and execution contexts—the mechanisms that con
 
 == The Executor Concept
 
-An *executor* is an object that can schedule work for execution. In Capy, executors must provide two methods:
+An *executor* is an object that can schedule work for execution. In Capy, executors must provide three methods:
 
 [source,cpp]
 ----

doc/modules/ROOT/pages/8.design/8l.RunApi.adoc

@@ -232,7 +232,7 @@ Coroutine frame allocation happens _before_ the coroutine body executes. When th
 
 Any mechanism that injects the allocator _after_ the call -- receiver queries, `await_transform`, explicit method calls -- arrives too late. The frame is already allocated.
 
-This is the fundamental tension identified in D4003 �3.3:
+This is the fundamental tension identified in D4003 section 3.3:
 
 [quote]
 ____

doc/modules/ROOT/pages/why-capy.adoc

@@ -408,9 +408,9 @@ No other solution like this exists. Not Asio. Not `std::execution`. Not anywhere
 
 == The Road Ahead
 
-For twenty five years, Boost.Asio has stood alone. It defined what portable asynchronous I/O looks like in {cpp}. No serious competitor offering its depth of offerings has appeared. It defined the promising Networking TS. Asio earned its place through years of production use, careful evolution, and relentless focus on real problems faced by real developers.
+For twenty-five years, Boost.Asio has stood alone. It defined what portable asynchronous I/O looks like in {cpp}. No serious competitor offering its depth of offerings has appeared. It defined the promising Networking TS. Asio earned its place through years of production use, careful evolution, and relentless focus on real problems faced by real developers.
 
-Capy builds on Asio's foundation—the buffer sequences, the executor model, the hard-won lessons about what works. But where Asio must preserve compatibility with over decades of existing code, Capy is free to commit fully to the future. {cpp}20 coroutines are not an afterthought here. They are the foundation.
+Capy builds on Asio's foundation—the buffer sequences, the executor model, the hard-won lessons about what works. But where Asio must preserve compatibility with decades of existing code, Capy is free to commit fully to the future. {cpp}20 coroutines are not an afterthought here. They are the foundation.
 
 The result is something new. Stream concepts designed for coroutines alone. Type-erasure at the level where it matters most. A simple execution model discovered through use-case-first design. Clean separation between execution and platform. A taxonomy of awaitables that invites extension rather than mandating a single concrete type.