Show `Implementing a RandomVariable` as the last entry in `How-to`

[ricardoV94]

Description

Or even move it to another section. Right now it will be the first thing suggested after the PR step-by-step guide

[OmGhadge]

Hi , I fixed the issue but since building docs is not supported on windows I am trying to run pymc in docker but I'm facing some issue . I've provided details about the problem in a post on the PyMC discussion forum, and I would greatly appreciate your guidance. -https://discourse.pymc.io/t/trouble-building-pymc-docker-image-on-windows/13654?u=omghadge.

[ricardoV94]

@OmGhadge if you open a PR there will be a preview, so maybe it will just work out of the box and you will be able to check on the preview