Update Literate requirement to 2.21.0 in /docs

[dependabot]

Updates the requirements on Literate to permit the latest version.

Release notes

Sourced from Literate's releases.

v2.21.0

See CHANGELOG.md for notable changes.

See CHANGELOG.md for notable changes.

Diff since v2.20.1

Changelog

Sourced from Literate's changelog.

[v2.21.0] - 2025-11-19

Changed

• Allow IOCapture version 1. (#280)

[v2.20.1] - 2024-10-17

Fixed

• Errors from code evaluation (with continue_on_error = true) are now properly displayed with showerror. (#261)

[v2.20.0] - 2024-10-16

Added

• A new keyword argument configuration continue_on_error::Bool = false has been added which controls the behavior of code execution errors. By default (continue_on_error = false) execution errors are re-thrown by Literate (as before). If continue_on_error = true is set the error is used as the block result and execution continues with following blocks. (#201, #257)
• Literate now replaces Documenter-style admonitions when generating notebook output (#259). Concretely,

  # !!! note
  #     A note.
  !!! warn "Warning title text"
  A warning.
  

  is replaced with

  # > **Note**
  # >
  # > A note.
  > Warning title text
  >
  > A warning.
  

[v2.19.1] - 2024-09-13

Fixed

• Set :SOURCE_PATH in the task local storage to the output file when executing code so that recursive include works as expected. (#251, #252)

[v2.19.0] - 2024-07-11

Changed

• Literate.markdown, Literate.notebook, and Literate.script are marked as public in Julia version that support the public keyword. (#248)

[v2.18.0] - 2024-04-17

Added

• Literate will now remove DocumenterInterlinks.jl

... (truncated)

Commits

• a86869f Release 2.21.0 (#281)
• 5b41365 Support IOCapture 1.x (#280)
• 8bde981 Bump actions/checkout from 4 to 5 (#279)
• da4c384 Clarify return values. (#277)
• 0d308e4 Update README links
• 5ea086c Add formatting commit to blame ignore list
• 5d1b491 Apply Runic formatting (#266)
• 51cee54 Bump codecov/codecov-action from 4 to 5 (#265)
• 5c15d88 Set version to 2.20.1.
• c4ac614 Use showerror for showing captured errors (#261)
• Additional commits viewable in compare view