docs: fix broken code examples and stale references across the doc set

[jmdelouis]

Summary

Audited every doc page against the actual source (healpix_plot/*.py) and fixed
everything that was factually wrong or would error if a user copy-pasted it.

• plot-function.md: clarify that ax passed to plot() must be a Cartopy
  GeoAxes created with subplot_kw={"projection": ...} — a plain
  plt.subplots() axis raises AttributeError: 'Axes' object has no attribute 'set_extent'. Added a "Multiple panels side by side" example, and documented
  the previously-missing view and rgb_clip parameters.
• mollview.md: fixed all 7 code examples importing from a nonexistent
  mollview module (from mollview import ... → from healpix_plot.mollview import ...); corrected the ellipsoid section, which claimed
  healpix_geo.nested.lonlat_to_healpix is always used (it's
  healpix_geo.ring unless nest=True).
• sampling-grid.md: fixed the bbox example calling
  healpix_plot.SamplingGrid.from_bbox(...), which doesn't exist —
  from_bbox is a classmethod of ParametrizedSamplingGrid.
• healpix-grid.md: added the missing zuniq caveat (as_keyword_params()
  always passes depth, which healpix_geo.zuniq.healpix_to_lonlat doesn't
  accept).
• README.md: package name was healpix-plotting in 4 places (including
  the pip install command) vs. the real healpix-plot; fixed a dead
  #healpixgrid anchor link.
• Aligned the Python version requirement to >= 3.11 (per pyproject.toml)
  across README.md, docs/installation.md, and mollview.md, which
  previously disagreed (3.13 / 3.13 / 3.10).

Test plan

• grep -rn "healpix-plotting\|from mollview import\|SamplingGrid.from_bbox" docs/ README.md returns no false positives
• Build the docs site and check the new admonitions/anchors render
• Manually run the updated "Multiple panels side by side" example

[jmdelouis]

@keewis To review ?

[keewis]

looks good to me, but I have two comments. The first can be a separate PR (so doesn't have to be addressed here), but the second is a very common LLM-ism comparing to the previous version that I'd just remove.

[keewis]

that's a bug in the implementation, so I'd argue we should rather improve our code. Switching from the internal dispatching to healpix_geo.auto should get us most of the way.