docs: fix typos and improve testing documentation (#4273)

* docs: fix typos and improve testing documentation

- Fix multiple typos in MAPDL documentation (madpl -> mapdl)
- Improve testing documentation with environment variables table
- Add DMP and SMP terms to dictionary
- Minor code comment fixes

* chore: adding changelog file 4273.documentation.md [dependabot-skip]

* Apply suggestions from code review

Co-authored-by: Copilot <[email protected]>

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Co-authored-by: pyansys-ci-bot <[email protected]>
Co-authored-by: Copilot <[email protected]>

Author: 3 people

doc/changelog.d/4273.documentation.md

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+Fix typos and improve testing documentation