Add Rails console docs

docs/how-to/access--the-rails-console.md

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+# How to access the Rails console
+
+Ideally, the discourse charm should be handled by interacting with Juju, and everything that can be done with the console should
+be doable with juju actions. In case there is something that needs to be done with the console anyway, the console can be accessed.
+
+### Prerequisites
+
+Have a Discourse deployment active.
+
+### Access the console
+
+First of all, switch into the juju Discourse model if necessary:
+```
+juju switch discourse-model
+```
+Then ssh into the Discourse unit:
+```
+juju ssh --container discourse discourse-k8s/leader bash   
+```
+Now populate the required environment variables, this requires charm revision 70 or greater:
+```
+sudo -u _daemon_ xargs -0 -L1 -a "/proc/$(ps aux | awk '/[u]nicorn master/ {print $2;exit}')/environ" | awk '{FS="=";print "export " $1 "=\"" $2 "\""}' > /root/envi && . /root/envi && rm /root/envi
+```
+Now change to the appropriate directory and execute the command to access the console:
+```
+cd /srv/discourse/app
+RAILS_ENV=production bin/bundle exec rails console
+```
+If the output of the last command contains something similar to this:
+```
+Loading production environment (Rails 7.0.5.1)
+irb(main):001:0>
+```
+Congratulations, you have accessed the rails console.