📚 DOCS: Add web badge

[choldgraf]

This adds a web badge and some instructions for how others can use it. It uses a base64-encoded image that generated the badge via shields.io

closes executablebooks/meta#156

[codecov]

Codecov Report

Merging #1380 (8bb10b8) into master (28f952d) will decrease coverage by 1.02%.
The diff coverage is n/a.

❗ Current head 8bb10b8 differs from pull request most recent head 1360b37. Consider uploading reports for the commit 1360b37 to get more accurate results

@@            Coverage Diff             @@
##           master    #1380      +/-   ##
==========================================
- Coverage   91.07%   90.04%   -1.03%     
==========================================
  Files           7        7              
  Lines         672      663       -9     
==========================================
- Hits          612      597      -15     
- Misses         60       66       +6     

Flag	Coverage Δ
pytests	90.04% <ø> (-1.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
jupyter_book/cli/pluggable.py	42.42% <0.00%> (-12.13%)	⬇️
jupyter_book/sphinx.py	85.71% <0.00%> (-1.96%)	⬇️
jupyter_book/cli/main.py	94.27% <0.00%> (-0.77%)	⬇️
jupyter_book/config.py	93.52% <0.00%> (ø)
jupyter_book/__init__.py	100.00% <0.00%> (ø)

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[damianavila]

There is a misalignment between the logo and the text on the y-axis, IMHO.
But that happens in other places as well, so it is probably an issue to solve in another PR.
LGTM.

[choldgraf]

Ya i noticed that too, but wasn't sure how to fix it since we are using shields to generate it 🤔