v0.13.6

ApproxFun v0.13.6

Diff since v0.13.5

Merged pull requests:

• Move show methods to ApproxFunBase (#784) (@jishnub)
• add docs build workflow (#787) (@jishnub)
• limit docstring of values to Fun` (#788) (@jishnub)
• Version bump to v0.13.6 (#789) (@jishnub)