[devtools] Implement new document structure in Pantheon and git (follow up on 1353)

= Mockup, docs are live, see

https://access.redhat.com/documentation/en-us/red_hat_developer_tools/1/

TODO: (wip list)

• each toolset own section in splash page
• versioning of toolsets are kept in document title
  • TODO edit headings to contain versions

• for 2019.1 2019.2 2019.3 (last 3 versions) links are manually added into splash page manager

• each toolset should have it's own branch
  • llvm_<version>
  • rust_<version>
  • go_<version>
  • eclipse_<version>
    and each toolset should have stage/prod branch as well, so we are talking about 3*4*2= 24 branches here

:prod-ver: has to be removed from compilers eventually. or edit from 20xx.x to respective compiler/eclipse version (+ add individual attribute file for each compiler (comp-ver is there though, investigate)