Start documenting autodiff activities

[ZuseZ4]

@kevinyamauchi A first draft, please lmk whether they answer your question, or where you'd like clarifications or rewording, to make it easier to understand.

r? ghost

[rust-log-analyzer]

The job tidy failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

finished building tool typos
error: `unter` should be `under`
     ╭▸ library/core/src/macros/mod.rs:1520:47
     │
1520 │     /// Related information can also be found unter the term "Vector-Jacobian product" (VJP).
     ╰╴                                              ━━━━━
error: `unter` should be `under`
     ╭▸ library/core/src/macros/mod.rs:1567:47
     │
1567 │     /// Related information can also be found unter the term "Jacobian-Vector Product" (JVP).
     ╰╴                                              ━━━━━
rerun tidy with `--extra-checks=spellcheck --bless` to fix typos
tidy [extra_checks]: checks with external tool 'typos' failed
tidy [extra_checks]: FAIL
tidy: The following check failed: extra_checks
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 /node/bin/npm --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1549:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1280:29

Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:02:37
  local time: Tue Oct 28 07:57:42 UTC 2025
  network time: Tue, 28 Oct 2025 07:57:42 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"