[FW][IMP] mrp: update variants doc

[fw-bot]

This PR is an [IMP] targeting the product_variants doc in Odoo's MRP documentation.

Performed complete rewrite to the document for 14.0 to be fwd-ported (targeted revisions for 15.0 and 16.0 to follow).

• Completely rewrote the .rst file and added example flow instructions
• Added additional screenshots
• Rewrote and added to doc to make more robust and comprehensive than before

Forward-Port-Of: #5236

[robodoo]

Pull request status dashboard.

[fw-bot]

@brse-odoo @StraubCreative cherrypicking of pull request #5236 failed.

stdout:

Auto-merging content/applications/inventory_and_mrp/manufacturing/management/product_variants.rst
CONFLICT (content): Merge conflict in content/applications/inventory_and_mrp/manufacturing/management/product_variants.rst

stderr:

02:56:29.194339 git.c:463               trace: built-in: git cherry-pick 940d9e02c857d272abe64c1ce3ba46c512daad82
error: could not apply 940d9e02c... [IMP] mrp: update variants doc
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
----------
status:

Either perform the forward-port manually (and push to this branch, proceeding as usual) or close this PR (maybe?).

In the former case, you may want to edit this PR message as well.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port

[StraubCreative]

@samueljlieber @brse-odoo the entire fwp doc has merge conflicts— please work together and advise to what's best for 15.0+, thanks!

[samueljlieber]

Resolved conflict in 7a747c9 due to past small changes in the 15.0 version of product_variants.rst. I cherry-picked the improvements in #5236. These changes should be fine to FWP through to master.

[brse-odoo]

@robodoo r+