SPI validation fixes

[anth-volk]

Fixes #918.
Fixes #919.

This PR raises the PolicyEngine UK model's convergence with the SPI's 2020/21 data. This PR also introduces a documentation page (currently a draft requiring some clarification) that describes the validation exercise, as well as areas where the two models diverge.

[anth-volk]

Absolute error measurement as of commit c2039ef:

<=10      91.461733
<1000      5.525746
>=1000     1.865212
<100       1.147309

[anth-volk]

Absolute error measurement as of commit 581569b:

<=10      92.685731
<1000      4.806584
>=1000     1.671160
<100       0.836525

[anth-volk]

Absolute error measurement as of ee35212:

<=10      92.695427
<1000      4.807466
>=1000     1.670153
<100       0.826954

[anth-volk]

Absolute error as of c974b42:

<=10      92.685731
<1000      4.806584
>=1000     1.671160
<100       0.836525