Bump version number to 0.13.0rc0

[josh146]

Context: Prepares for the 0.13.0 release candidate 0 release.

Description of the Change:

• Bumps version number to 0.13.0rc0

• Updates changelog with recent PRs, and corrects Python example

• Pins sphinx-gallery to version 0.5, as 0.6 was recently released, breaking the documentation CSS.

Benefits: n/a

Possible Drawbacks: n/a

Related GitHub Issues: n/a

[codecov[bot]]

Codecov Report

Merging #344 into master will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #344      +/-   ##
==========================================
+ Coverage   97.67%   97.70%   +0.03%     
==========================================
  Files          51       51              
  Lines        6355     6355              
==========================================
+ Hits         6207     6209       +2     
+ Misses        148      146       -2     

Impacted Files	Coverage Δ
strawberryfields/_version.py	100.00% <100.00%> (ø)
strawberryfields/api/connection.py	97.61% <100.00%> (ø)
strawberryfields/engine.py	95.45% <100.00%> (ø)
strawberryfields/backends/tfbackend/ops.py	96.08% <0.00%> (+0.34%)	:arrow_up:

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