Fix #2413, Update Dev Guide to fix out-of-date references and broken links

[thnkslprpt]

Checklist

• I reviewed the Contributing Guide.
• I signed and emailed the appropriate Contributor License Agreement to GSFC-SoftwareRelease@mail.nasa.gov and copied cfs-program@lists.nasa.gov.

Describe the contribution

• Fixes cFE Dev Guide has out-of-date references and some broken links #2413
  • Delete references to 'cfe_testrunner' module that was removed in Fix #1281, remove testrunner and convert testcase to app #1292
  • Update references to old function/variables: CFE_MSG_GetTime, CFE_TIME_VALID, CFE_TIME_INVALID, CFE_TIME_FLYWHEEL
  • A few typos etc. picked up along the way (and applied to other files at the same time)
  • Broken links in the Developer's Guide 'Table of Contents' fixed

Testing performed
GitHub CI actions all passing successfully (incl. Build + Run, Unit/Functional Tests etc.).

Expected behavior changes
Doc changes only.

Contributor Info
Avi Weiss @thnkslprpt