Fix broken links in dev docs

[radinamatic]

Summary

• https://pex.readthedocs.io/en/stable/ ✔️
• https://kolibri-dev.readthedocs.io/pipeline/frontend_build_pipeline ✔️
• https://kolibri-dev.readthedocs.io/en/develop/build_system/Pipeline_vocab_illustrated.png 🚫 - Will need to be targeted to develop?
• https://formatjs.io/docs/icu-syntax/ ✔️
• https://github.com/learningequality/kolibri-release-process/ - Added (private repo)
• https://github.com/learningequality/nutritionfacts/ - Added (private repo)
• https://virtualenv.pypa.io/en/stable/userguide/ ✔️
• http://django-mptt.github.io/django-mptt/ ✔️
• http://django-mptt.github.io/django-mptt/overview.html/ ✔️

Reviewer guidance

Build dev docs and check if the links are corrected properly.

References

Fixes #7807

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Contributor Checklist

PR process:

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Testing:

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

Reviewer Checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[jonboiser]

Looks good!