Rename "basic" directory in docs to "getting_started"

[santisoler]

Summary

Rename basic directory in the docs to getting_started.
Move the getting_started.rst file to getting_started/index.rst.
Fix links pointing to the old basic pages. Now they point to getting_started.

PR Checklist

• If this is a work in progress PR, set as a Draft PR
• Linted my code according to the style guides.
• Added tests to verify changes to the code.
• Added necessary documentation to any new functions/classes following the
  expect style.
• Added relevant method tags (i.e. GRAV, EM, etc.)
• Marked as ready for review (ff this is was a draft PR), and converted
  to a Pull Request
• Tagged @simpeg/simpeg-developers when ready for review.

[codecov]

Codecov Report

Merging #1250 (8322af8) into main (9e08237) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1250   +/-   ##
=======================================
  Coverage   82.31%   82.31%           
=======================================
  Files         164      164           
  Lines       24965    24965           
=======================================
  Hits        20549    20549           
  Misses       4416     4416