De-duplicate README content and update installation instructions (#274)

* Remove separate README for PyPI

* Use dynamic all contributors badge

* Use absolute URLs for images to make README portable

* Use Unicode emoji characters for portability

* Remove duplicated content about updated features

* Formatting tweaks

* Update installation instructions

* Tweak README formatting to make work when included in Sphinx docs

* Add note about GPU / TPU JAX installation

* Use top-level README as docs index page

* Add package name to header

* Remove old installation instructions from docs

* Update tutorials index page to be consistent with README

* Set version in docs automatically

Author: matt-graham

.pip_readme.rst

@@ -2,7 +2,6 @@ graft s2fft
 graft tests
 graft docs
 
-include .pip_readme.rst
 include README.rst
 include pytest.ini
 include LICENCE.txt