Add logo to the documentation

[hyrodium]

This PR adds a logo to the documentation.

I designed this logo with the following in mind:

• Arrange four types of Interval.
  • $\textcolor{#4063D8}{I_1}$ : OpenInterval
  • $\textcolor{#389826}{I_2}$ : ClosedInterval
  • $\textcolor{#9558B2}{I_3}$ : Interval{:closed, :open}
  • $\textcolor{#CB3C33}{I_4}$ : Interval{:open, :closed}
• Julia logo constructed of the closed endpoints of $\textcolor{#389826}{I_2}$, $\textcolor{#9558B2}{I_3}$, $\textcolor{#CB3C33}{I_4}$.
• Imply set operations such as $\textcolor{#CB3C33}{I_4} = \textcolor{#4063D8}{I_1} \cap \textcolor{#389826}{I_2}$.
• The colors are from julia-logo-graphics.

I hope everyone likes this logo!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d0c0100) 99.10% compared to head (d785520) 99.10%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #165   +/-   ##
=======================================
  Coverage   99.10%   99.10%           
=======================================
  Files           4        4           
  Lines         224      224           
=======================================
  Hits          222      222           
  Misses          2        2           

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[hyrodium]

@aplavin @dlfivefifty
Do you have any thoughts on the design?

[dlfivefifty]

Looks good!