Add docs for tool panel and panel view administration

[bernt-matthias]

Tool panel administration was always mysterious to me. I have always wondered

• which XML files am I supposed to touch (in particular where to add custom tools)
• how to obtain a useful ordering and add panels (modification of integrated_tool_conf.xml is what I often heard)
• how to keep tool_conf.xml up to date during updates. So far I used a copy of the sample file where I added custom tools. Updating this copy with the releases is error-prone.

I experimented a bit and added my experiences to the docs .. which were a bit outdated and in parts made statements about how things worked in the past (which was not so motivating read). For me, the main takeaways of my journey are: avoid touching any of tool_conf.xml, shed_tool_conf.xml, and integrated_tool_conf.xml, but instead

• define section ordering and labels in an extra tool conf that is listed first in tool_config_file
• directly include tool_conf.xml.sample in the tool_config_file configuration list (in this way built-in tools stay up to date)
• add custom tools to extra tool conf files.

The changes also contain the first attempt to document the tool conf XML syntax.

Finally, I discovered the so far undocumented (besides the PR description) tool panel views and added the documentation here (mainly copy paste from the PR).

How to test the changes?

(Select all options that apply)

• I've included appropriate automated tests.
• This is a refactoring of components with existing test coverage.
• Instructions for manual testing are as follows:
  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

License

• I agree to license these and all my past contributions to the core galaxy codebase under the MIT license.

[nsoranzo]

Great work!

[github-actions]

This PR was merged without a "kind/" label, please correct.