add envars + packaging docs, and hierarchy changes in doc

[pirat89]

Let's split description into 3 parts.

1. Added additional new documentation about envars and our solution of dependencies between the leapp framework and leapp repositories.

2. Reorg the structure of docs. I found the original structure very unfortunate from the point of el7toel8 repository. When we want to document stuff that is related just for the one specific repository, it's hard to find the right place where to put it. We have currently two tutorials, which one of them is already overcrowded by mixture of stuff related to the repository and actors from my POV. I am keeping the current tutorials kind of untouched - possible move of some parts can be done in future if needed. I am now super-busy and do not want to go so deep with the changes. Just picking main changes now:

   • create a leapp repositories entry on the top level menu, that should contains stuff related just to repositories; in case of docs specific only to one leapp-repository - now el7toel8 only - are grouped in subentries (keeping tutorials it's tutorials in tutorials + under the repo entry too)
   • added the Infrastructure entry. Where we document stuff related to dependencies/packaging, CI/CD related stuff, versioning... whatever from the point of the leapp. Maybe the name is not the best, but I think that this stuff should be really grouped more together instead of growing top level menu

3. structrure of the source files for documentations - the number of doc files is growing and to find specific files in one directory... So I would like to put some files under subdirs as well. e.g. files realted to el7toel8 are in their subdir. The links are obviously changed in that case. I already did some research where I should change links in external documents and I feel ok to do that.

To see the current shape of docs:

virtualenv --system-site-packages -p /usr/bin/python3.7 tut
. tut/bin/activate
pip install -r requirements-docs.txt
cd docs
make livehtml

and open 127.0.0.1:8000 in your browser.

---

The original "structure"

(hidden additional sub-trees that are not affected now)

.
├── Terminology
├── Tutorials
  ├── ... some generic tutorials
  ├── How to create a Leapp actor for RHEL 7 to 8 upgrade
  ├── How to properly inhibit the RHEL 7 to 8 upgrade process
  └── ... generic tutorials
├── Repository Directory Layout
├── Best Practices for actor developmemt
├── Tests for actors
├── Achitecture overview
├── How to deal with dependencies
├── How to deal with dependencies in leapp-repository
├── Inplace Upgrade Workflow
├── Contibuting to the Leapp project
├── Coding guidelines for the Leapp project
├── Frequenstly Asked Questions
└── Python Documentations for the leapp package

The structure proposed in this PR

(currently; there is more stuff to change yet, but rather waiting for discussion before I put my time into it)
Note: see that relevant tutorials have duplicit reference, to keep the tutorials list complete and have reference on them in specific subtree too.

.
├── Terminology
  └── ...
├── Tutorials
  ├── ... some generic tutorials
  ├── How to create a Leapp actor for RHEL 7 to 8 upgrade
  ├── How to properly inhibit the RHEL 7 to 8 upgrade process
  └── ... generic tutorials
├── Leapp repositories
  ├── Repository Directory Layout
  └── Leapp repository for RHEL 7 to RHEL 8 upgrade
    ├── How to create a Leapp actor for RHEL 7 to 8 upgrade
    ├── How to properly inhibit the RHEL 7 to 8 upgrade process
    └── Environment variables for el7toel8repository
├── Infrastructure
  ├── How to deal with dependencies
  ├── How to deal with dependencies in leapp-repository
  └── Compatibility between leapp and leapp repositories
├── Best Practices for actor developmemt
├── Tests for actors
├── Achitecture overview
├── Inplace Upgrade Workflow
├── Contibuting to the Leapp project
├── Coding guidelines for the Leapp project
├── Frequenstly Asked Questions
└── Python Documentations for the leapp package

[centos-ci]

Can one of the admins verify this patch?

[bocekm]

I've checked just the language, not if it easy to understand. And I haven't checked the hierarchy changes.

[pirat89]

Guys, I would like to know your opinions on the restructuralisation plans here as well @oamg/developers

[Rezney]

@pirat89 I like the hierarchy change idea. But for the moment it is very high-level to judge completely as soe content is duplicated (e.g. How to create a Leapp actor for RHEL 7 to 8 upgrade) and it is not completely clear what will go where...

[pirat89]

@bocekm Done.