Use Foreman Documentation for REX plugin

[maximiliankolb]

Users have complained that we still mention --enable-foreman-proxy-plugin-remote-execution-ssh instead of --enable-foreman-proxy-plugin-remote-execution-script in Foreman Manual. This PR removes the content and instead refers users to docs.theforeman.org.

It was wrong since Foreman 3.3.

Does it make sense to version the REX plugin at all? Or should I only make changes to "nightly"?

Please also note that I did not check every line myself; but for reference, the last contribute was from lzap in 2019; before that, the last correction was by Ewoud in 2019 as well.

See theforeman/foreman-documentation#1301 & https://community.theforeman.org/t/foreman-remote-execution-plugin-documentation/28889

[ekohl]

As discussed, we have documentation linking to it today:

$ rg -i 'documentation_?(url|button)'
app/views/job_invocations/welcome.html.erb
10:           documentation_url('1.ForemanRemoteExecution1.3Manual', :root_url => 'https://www.theforeman.org/plugins/foreman_remote_execution/1.3/index.html#'), :rel => 'external' %></p>

app/views/job_templates/index.html.erb
5:<% title_actions(documentation_button_rex('3.1JobTemplates'),

app/helpers/remote_execution_helper.rb
58:      documentation_button_rex('3.2ExecutingaJob'),
205:  def documentation_button_rex(section = '')
208:    documentation_button section, :root_url => url

webpack/__mocks__/foremanReact/constants.js
23:    documentationUrl: `4.1.5Searching`,

webpack/react_app/components/TargetingHosts/__tests__/__snapshots__/TargetingHostsPage.test.js.snap
28:              "documentationUrl": "4.1.5Searching",

[maximiliankolb]

Applied all feedback and rebased to gh-pages.