Misc doc fixes.

[Dirbaio]

• Switch from GAT to AFIT has already happened.
• We're no longer planning to integrate EHA into EH
• Fix all broken links.

[bors[bot]]

Build succeeded:

• clippy
• fmt
• test (1.59.0, thumbv6m-none-eabi)
• test (1.59.0, thumbv7m-none-eabi)
• test (1.59.0, x86_64-unknown-linux-gnu)
• test (nightly, thumbv6m-none-eabi)
• test (nightly, thumbv7m-none-eabi)
• test (nightly, x86_64-unknown-linux-gnu)
• test (stable, thumbv6m-none-eabi)
• test (stable, thumbv7m-none-eabi)
• test (stable, x86_64-unknown-linux-gnu)