Expand the README and the documentation landing page

[roytsmart]

The landing page had no
installation instructions and no runnable example, so the first executable code
a visitor met was inside an API docstring. The README's feature list had also
fallen behind the one in the documentation, missing image simulation, the
linear forward model, sensor simulation, glass materials, and the ruling
profiles.

Landing page

• An Installation section, which only the README had.
• An Examples section with two worked examples: the transmissivity of an
  aluminum filter, which shows the L edge that solar EUV instruments are
  designed around, and the effective quantum efficiency of a back-illuminated
  CCD compared against an ideal back surface.
• A pointer to the worked Newtonian telescope in
  optika.systems.SequentialSystem rather than a duplicate of it.
• The page title is now optika rather than Introduction.

I did try adding a compact raytrace figure here, but an on-axis mirror plotted
in the z-x plane draws the incoming and reflected legs on top of each other,
which reads worse than no figure at all. That is why the section links to the
telescope example instead of shipping a lesser version of it.

README

• The feature list now matches the documentation.
• A link to the documentation, which the README did not have.
• A Key concepts section covering what differs most from Zemax: surfaces
  are placed in global coordinates rather than relative to the previous
  surface, the field of view and entrance pupil are computed rather than
  specified, rulings are a property of a surface, and any parameter can be an
  array (which is also how uncertainty propagates).
• The filter example added to the gallery, and a development section including
  mypy, which this repository runs and its siblings do not.
• uncertainity is now spelled correctly.

Verification

Built the docs locally: exit code 0, both new figures generated, and no
warnings referencing index.rst. The only warnings in the build are
pre-existing ones from the package's own docstrings (an \O escape sequence in
the depletion models, and astropy's note about sqrt on a Quantity
subclass).

Both examples were also run standalone before being added, at roughly 5 seconds
each, and both figures were inspected.

The new figures are index_0_0.png and index_1_0.png. The README's existing
gallery links point at docstring-generated images, so none of them are affected
by this change.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.58%. Comparing base (9ff5a48) to head (45e81a1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #200 +/- ##
=======================================
Coverage 99.58% 99.58% =======================================
Files 118 118 Lines 6933 6933 =======================================
Hits 6904 6904 Misses 29 29 

Flag	Coverage Δ
unittests	99.58% <ø> (ø)

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