Add uconvert and chemistry example

[gaurav-arya]

I was doing my chemistry homework using DynamicQuantities.jl, and thought it might make for a nice docs example! This is on top of #47, making the diff a bit noisier.

Edit: This PR now also implements uconvert, see #48 (comment).

[github-actions]

Benchmark Results

	main	4c34768...	t[main]/t[4c34768...]
Quantity/creation/Quantity(x)	3.1 ± 0.1 ns	3.4 ± 0 ns	0.912
Quantity/creation/Quantity(x, length=y)	3.4 ± 0 ns	3.7 ± 0 ns	0.919
Quantity/with_numbers/*real	3.4 ± 0 ns	3.4 ± 0 ns	1
Quantity/with_numbers/^int	11.2 ± 3.4 ns	11.5 ± 4.1 ns	0.974
Quantity/with_numbers/^int * real	11.8 ± 3.9 ns	12.1 ± 4.3 ns	0.975
Quantity/with_quantity/+y	7.7 ± 0.001 ns	6.8 ± 0.1 ns	1.13
Quantity/with_quantity//y	4 ± 0 ns	3.7 ± 0.1 ns	1.08
Quantity/with_self/dimension	1.7 ± 0 ns	1.7 ± 0 ns	1
Quantity/with_self/inv	3.4 ± 0 ns	3.7 ± 0 ns	0.919
Quantity/with_self/ustrip	1.7 ± 0 ns	1.7 ± 0 ns	1
QuantityArray/broadcasting/multi_array_of_quantities	0.226 ± 0.025 ms	0.226 ± 0.025 ms	1
QuantityArray/broadcasting/multi_normal_array	0.0782 ± 0.005 ms	0.0762 ± 0.0018 ms	1.03
QuantityArray/broadcasting/multi_quantity_array	0.255 ± 0.0014 ms	0.255 ± 0.0013 ms	0.999
QuantityArray/broadcasting/x^2_array_of_quantities	0.0421 ± 0.0035 ms	0.0434 ± 0.0031 ms	0.97
QuantityArray/broadcasting/x^2_normal_array	9.1 ± 1.8 μs	9.3 ± 1.8 μs	0.979
QuantityArray/broadcasting/x^2_quantity_array	10 ± 1.1 μs	10 ± 1.2 μs	1
QuantityArray/broadcasting/x^4_array_of_quantities	0.133 ± 0.0035 ms	0.136 ± 0.0058 ms	0.979
QuantityArray/broadcasting/x^4_normal_array	0.0688 ± 0.0008 ms	0.0692 ± 0.0014 ms	0.994
QuantityArray/broadcasting/x^4_quantity_array	0.0981 ± 0.0028 ms	0.103 ± 0.0015 ms	0.95
time_to_load	0.169 ± 0.0017 s	0.169 ± 0.0021 s	1

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).

[gaurav-arya]

Remove string method

[MilesCranmer]

Looks good! Great work @gaurav-arya!!