chore(deps): update myst-parser requirement in /requirements

Updates the requirements on [myst-parser](https://github.com/executablebooks/MyST-Parser) to permit the latest version.
- [Release notes](https://github.com/executablebooks/MyST-Parser/releases)
- [Changelog](https://github.com/executablebooks/MyST-Parser/blob/master/CHANGELOG.md)
- [Commits](executablebooks/MyST-Parser@v2.0.0...v5.1.0)

---
updated-dependencies:
- dependency-name: myst-parser
  dependency-version: 5.1.0
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements/documentation.txt

@@ -1,7 +1,7 @@
 # Documentation (for devs)
 # -----------------------
 
-myst-parser[linkify]>=2
+myst-parser[linkify]>=5.1.0
 sphinx-autobuild>=2024
 sphinx-copybutton>=0.5,<1
 sphinx-rtd-theme>=2