[doc] Rust for C devs: Outdated information and improvements

### Description

While the [Rust for Embedded C Programmers](https://opentitan.org/book/doc/rust_for_c_devs.html) seems well written and of high quality, there are a few enhancements that could be done:

- [Pointers](https://opentitan.org/book/doc/rust_for_c_devs.html#pointers) section: \
Present [`addr_of`](https://doc.rust-lang.org/stable/core/ptr/macro.addr_of.html) and [`addr_of_mut`](https://doc.rust-lang.org/stable/core/ptr/macro.addr_of_mut.html) as alternatives to `&x as *const T`.

- [Macros](https://opentitan.org/book/doc/rust_for_c_devs.html#macros) section:\
Link to https://veykril.github.io/tlborm/ in addition or instead of https://danielkeep.github.io/tlborm/book/index.html

- [Inline Assembly](https://opentitan.org/book/doc/rust_for_c_devs.html#inline-assembly) section:\
`llvm_asm` has now been removed and [`asm`](https://doc.rust-lang.org/stable/core/arch/macro.asm.html#) is stable since Rust 1.59

Some additional nitpicks:
- The `let else` syntax could also be presented, probably in the [Pattern Matching](https://opentitan.org/book/doc/rust_for_c_devs.html#pattern-matching) section.
- The link in footnote 71 is not clickable.

Open the Rust for Embedded C Programmers page and review the Pointers, Macros, Inline Assembly, Pattern Matching, and footnote 71 sections. Check the linked Rust references and update the outdated or missing information described in the issue, including the links and let-else example; done means all listed improvements are reflected in the rendered document.

Written by the indexing model from the issue text.