Prepare 3.1-8 release

[vincentvanhees]

In this PR:

• Improves vignette for reading custom csv files in response to Custom CSV fails with header #1237
• Splits long markdown table in GGIRoutput vignette to ease tracking future changes via git, fixes GGIRoutput.Rmd => split long tables to ease maintenance #1231
• Update vignette and documentation to clarify "+invalid" in guider name, fixes clarify '+invalid' in guider name #1217
• Various small vignette updates in order to wrap up restructuring of vignettes, fixes Restructure GGIR vignettes #1046

Checklist before merging:

• Existing tests still work (check by running the test suite, e.g. from RStudio).
• Added tests (if you added functionality) or fixed existing test (if you fixed a bug).
• Clean code has been attempted, e.g. intuitive object names and no code redundancy.
• Documentation updated:
  • Function documentation
  • Chapter vignettes for GitHub IO
  • Vignettes for CRAN
• Corresponding issue tagged in PR message. If no issue exist, please create an issue and tag it.
• Updated release notes in inst/NEWS.Rd with a user-readable summary. Please, include references to relevant issues or PR discussions.
• Added your name to the contributors lists in the DESCRIPTION file, if you think you made a significant contribution.
• GGIR parameters were added/removed. If yes, please also complete checklist below.