Add links to std and docs.rs (#2403)

- **Link to relevant crate APIs**
- **Add missing std links**

src/bare-metal/aps/mmio.md

@@ -1,10 +1,14 @@
 # Volatile memory access for MMIO
 
-- Use `pointer::read_volatile` and `pointer::write_volatile`.
+- Use [`pointer::read_volatile`] and [`pointer::write_volatile`].
 - Never hold a reference.
-- `addr_of!` lets you get fields of structs without creating an intermediate
+- Use [`addr_of!`] to get fields of structs without creating an intermediate
   reference.
 
+[`pointer::read_volatile`]: https://doc.rust-lang.org/stable/core/primitive.pointer.html#method.read_volatile
+[`pointer::write_volatile`]: https://doc.rust-lang.org/stable/core/primitive.pointer.html#method.write_volatile
+[`addr_of!`]: https://doc.rust-lang.org/stable/core/ptr/macro.addr_of.html
+
 <details>
 
 - Volatile access: read or write operations may have side-effects, so prevent

src/error-handling/anyhow.md

@@ -4,13 +4,16 @@ minutes: 5
 
 # `anyhow`
 
-The [`anyhow`](https://docs.rs/anyhow/) crate provides a rich error type with
-support for carrying additional contextual information, which can be used to
-provide a semantic trace of what the program was doing leading up to the error.
+The [`anyhow`] crate provides a rich error type with support for carrying
+additional contextual information, which can be used to provide a semantic trace
+of what the program was doing leading up to the error.
 
-This can be combined with the convenience macros from `thiserror` to avoid
+This can be combined with the convenience macros from [`thiserror`] to avoid
 writing out trait impls explicitly for custom error types.
 
+[`anyhow`]: https://docs.rs/anyhow/
+[`thiserror`]: https://docs.rs/thiserror/
+
 ```rust,editable,compile_fail
 use anyhow::{bail, Context, Result};
 use std::fs;

src/error-handling/exercise.md

@@ -7,7 +7,10 @@ minutes: 30
 The following implements a very simple parser for an expression language.
 However, it handles errors by panicking. Rewrite it to instead use idiomatic
 error handling and propagate errors to a return from `main`. Feel free to use
-`thiserror` and `anyhow`.
+[`thiserror`] and [`anyhow`].
+
+[`thiserror`]: https://docs.rs/thiserror
+[`anyhow`]: https://docs.rs/anyhow
 
 HINT: start by fixing error handling in the `parse` function. Once that is
 working correctly, update `Tokenizer` to implement