Add alchemical network creation userguide page

[IAlibay]

Fixes #805 #790

Checklist

• [ ] Added a news entry

Developers certificate of origin

• [x] I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 92.31%. Comparing base (0dd3a73) to head (2bb2d4e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #808 +/- ## ========================================== - Coverage 94.12% 92.31% -1.82% ========================================== Files 133 133 Lines 9767 9767 ========================================== - Hits 9193 9016 -177 - Misses 574 751 +177 ``` | [Flag](https://app.codecov.io/gh/OpenFreeEnergy/openfe/pull/808/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OpenFreeEnergy) | Coverage Δ | | |---|---|---| | [fast-tests](https://app.codecov.io/gh/OpenFreeEnergy/openfe/pull/808/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OpenFreeEnergy) | `92.31% <ø> (?)` | | | [slow-tests](https://app.codecov.io/gh/OpenFreeEnergy/openfe/pull/808/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OpenFreeEnergy) | `?` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OpenFreeEnergy#carryforward-flags-in-the-pull-request-comment) to find out more.

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[IAlibay]

Something's up with the cookbooks, for some reason the references aren't working. I suspect I won't have much time to hunt this one down, can someone else look into this?