Add note on tangent type implementation

[willtebbutt]

Adds a note on the implementation of tangent_type and related functions. No version bump required because it's all in the docs.

[github-actions]

Performance Ratio:
Ratio of time to compute gradient and time to compute function.
Warning: results are very approximate! See here for more context.

┌────────────────────────────┬──────────┬─────────┬─────────────┬─────────┐
│                      Label │ Mooncake │  Zygote │ ReverseDiff │  Enzyme │
│                     String │   String │  String │      String │  String │
├────────────────────────────┼──────────┼─────────┼─────────────┼─────────┤
│                   sum_1000 │     79.4 │     1.0 │         5.5 │    8.11 │
│                  _sum_1000 │     6.71 │  1570.0 │        34.3 │    1.09 │
│               sum_sin_1000 │     2.25 │    1.68 │        10.8 │    1.97 │
│              _sum_sin_1000 │     2.71 │   307.0 │        13.2 │    2.41 │
│                   kron_sum │     57.7 │    3.53 │       211.0 │    8.24 │
│              kron_view_sum │     57.3 │    8.99 │       237.0 │    97.0 │
│      naive_map_sin_cos_exp │     2.49 │ missing │        7.12 │    2.34 │
│            map_sin_cos_exp │      2.7 │    1.43 │        6.07 │    2.89 │
│      broadcast_sin_cos_exp │     2.55 │    2.23 │        1.47 │    2.25 │
│                 simple_mlp │     4.97 │    3.14 │        6.78 │    3.46 │
│                     gp_lml │     12.6 │    10.1 │     missing │    9.75 │
│ turing_broadcast_benchmark │     3.16 │ missing │        25.8 │ missing │
│         large_single_block │     4.42 │  4130.0 │        31.1 │    2.21 │
└────────────────────────────┴──────────┴─────────┴─────────────┴─────────┘

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

[sunxd3]

LGTM! very clear.

I would love to see an example of the Effects Etc section, but probably for later.

[willtebbutt]

Thanks @sunxd3 -- I've fixed a couple of typos that I've missed, and will now merge.