prepare space for fused docs

[Borda]

What does this PR do?

Make a dummy doc and set up related CI/CD jobs...
For preserving continuity, we use symlinks to the existing pages. When all is settled up, we will project the real pages via link to the new location and sunset the old docs location.
The cleaning requirements are also separated in #14216

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Did you have fun?

This is a preparation stage for docs unification. If you want to edit/update docs for ongoing documentation, do it in source-app or source-pytorch. Otherwise, if the changes are related to the future/fused docs, please rename/move the existing symlink or make a new file where you can still copy-paste the page content...

cc @Borda

[Borda]

ok, one remaining issue lest is with unification of templates:

Extension error (pt_lightning_sphinx_theme.extensions.lightning):
Handler <function process_card_nodes at 0x7f0acfce40d0> for event 'doctree-resolved' threw an exception (exception: 'BuildEnvironment' object has no attribute 'all_cardnodes')
make: *** [Makefile:19: html] Error 2

[akihironitta]

(I guess this has been discussed already somewhere and/or can be discussed/polished later in follow-up PRs, but)

IMHO, mixing both app and pl and having both at the same directory level seems confusing, e.g.

docs/source-lit/accelerators  # <- pl
docs/source-lit/strategies    # <- pl
...
docs/source-lit/code_samples  # <- app
docs/source-lit/workflows     # <- app
...

[Borda]

IMHO, mixing both app and pl and having both at the same directory level seems confusing, e.g.

This is just init state, and all shall be restructured by @Felonious-Spellfire in following PRs