Colab tutorial clarity

A user ran the whole notebook top to bottom, not realizing the sections
after installation are alternatives. Changes:

• Callout after the install section announcing that what follows is a
  choice, and headings renamed to Option A/B/C so the TOC shows it too.
• Option C (Google Drive) reframed as orthogonal — it applies to whichever
  option you picked — and shows both variants instead of silently repeating
  Option A.
• Note in Option B about ending an existing session before gj.init()
  switches to another project.
• Short definition of what a GRASS project is, linking to the
  manual,
  since "sample dataset" suggested a folder of GeoTIFFs.
• Fixed gs.init() → gj.init(), which raised AttributeError
  (init is not exported from grass.script).
• Typo: "bare in mind" → "bear in mind".

New sample project

Both tutorials now use raleigh_northcarolina_usa_epsg6542. elevation
and landuse carry over, but two cells in the fast track needed changes:

• r.univar -t ... separator=comma into pd.read_csv failed, because the
  NLCD class labels contain commas (Developed, Open Space). Switched to
  the pipe separator. (format="json" would be the robust fix, but
  r.univar gained it in 8.5 and Colab's ubuntugis PPA is still on 8.4.2.)
• The census vector doesn't exist in the new project; replaced with
  census_blocks and its POP20 / HOUSING20 columns.
• Regenerated grass_python_histogram.png from the new elevation raster.
  This also corrects the legend, whose labels were swapped in the old figure.

All cells in both tutorials were run against a fresh download of the new
project.