(core) freshen core README; support python3 in grist-core docker image

Summary:
This updates the grist-core README to list specific features of Grist,
to make it easier for a casual visitor to get a sense of its scope. Adds links
to some new resources (reviews, templates, grist v airtable post) that could
also help. Adds python3 to docker image so that templates work without fuss.

Test Plan: existing tests should pass

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: dsagal, anaisconce

Differential Revision: https://phab.getgrist.com/D3204

package.json

@@ -8,6 +8,8 @@
   "scripts": {
     "start": "tsc --build -w --preserveWatchOutput & catw app/client/*.css app/client/*/*.css -o static/bundle.css -v & webpack --config buildtools/webpack.config.js --mode development --watch --hide-modules & NODE_PATH=_build:_build/stubs nodemon --delay 1 -w _build/app/server -w _build/app/common _build/stubs/app/server/server.js & wait",
     "install:python": "buildtools/prepare_python.sh",
+    "install:python2": "buildtools/prepare_python2.sh",
+    "install:python3": "buildtools/prepare_python3.sh",
     "build:prod": "tsc --build && webpack --config buildtools/webpack.config.js --mode production && webpack --config buildtools/webpack.check.js --mode production && cat app/client/*.css app/client/*/*.css > static/bundle.css",
     "start:prod": "NODE_PATH=_build:_build/stubs node _build/stubs/app/server/server.js",
     "test": "GRIST_SESSION_COOKIE=grist_test_cookie GRIST_TEST_LOGIN=1 TEST_SUPPORT_API_KEY=api_key_for_support NODE_PATH=_build:_build/stubs mocha _build/test/nbrowser/*.js",