Add documentation for rec-space kenetic energy preconditioning

[Chengcheng-Xiao]

As title. This PR also updates the version and release to 8.0.0!

Here is a compiled PDF version: ONETEP_Documentation.pdf

Tagging @nickhine

[JacekDziedzic]

Thanks so much, @Chengcheng-Xiao, that's very helpful.

I only have a few very minor comments:

s/by default use/by default uses/
s/where,/where/

In the section header "3.17.1" replace k_0 with an actual subscript.

Next page:

s/kintetic/kinetic/
s/upreconditioning/preconditioning/
s/Basic string/Basic, string/ (twice)

k_zero is "physical", not "float". In the description change k0 to $k_0$.
In the description for precond_array change k0 to $k_0$.

[Chengcheng-Xiao]

@JacekDziedzic Thanks, done!