Add example on using TypedPolynomials

[blegat]

There are currently 3 issues blocking this:

• [x] https://github.com/jump-dev/SumOfSquares.jl/pull/241
• [x] The remainder obtained is not zero
• [x] I get a StackOverflow if I don't use polynomial on the last line

[codecov[bot]]

Codecov Report

Merging #242 (17c2581) into master (6443b92) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #242   +/-   ##
=======================================
  Coverage   78.17%   78.17%           
=======================================
  Files          36       36           
  Lines        1700     1700           
=======================================
  Hits         1329     1329           
  Misses        371      371           

Impacted Files	Coverage Δ
src/sosdec.jl	92.85% <ø> (ø)
src/gram_matrix.jl	93.93% <100.00%> (ø)

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