Update index page introduction

[djhoese]

As mentioned in #730 I felt that the introduction paragraph wasn't very helpful to new users or as a good description of its features.

• Closes Rewrite introduction paragraph in documentation #730
• Fully documented

[codecov]

Codecov Report

Merging #747 into master will increase coverage by 0.92%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #747      +/-   ##
==========================================
+ Coverage   80.69%   81.61%   +0.92%     
==========================================
  Files         149      157       +8     
  Lines       21649    22636     +987     
==========================================
+ Hits        17469    18475    +1006     
+ Misses       4180     4161      -19

Impacted Files	Coverage Δ
satpy/readers/modis_l1b.py	20% <0%> (-10.91%)	⬇️
satpy/readers/geocat.py	87.5% <0%> (-4.23%)	⬇️
satpy/node.py	91.42% <0%> (-2.86%)	⬇️
satpy/readers/goes_imager_nc.py	63.88% <0%> (-1.86%)	⬇️
satpy/readers/yaml_reader.py	93.03% <0%> (-0.33%)	⬇️
satpy/readers/grib.py	79.86% <0%> (-0.29%)	⬇️
satpy/tests/test_yaml_reader.py	98.51% <0%> (-0.26%)	⬇️
satpy/tests/reader_tests/test_virr_l1b.py	97.16% <0%> (-0.03%)	⬇️
satpy/writers/__init__.py	85.21% <0%> (ø)	⬆️
satpy/tests/reader_tests/test_modis_l1b.py
... and 32 more

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[coveralls]

Coverage increased (+0.9%) to 81.618% when pulling a821870 on djhoese:doc-index-summary into 1f21d20 on pytroll:master.

[sfinkens]

One little comment, otherwise nice work!

[sjoro]

looks good to me.

[mraspaud]

LGTM