Reorder cookbook sections and update style

Summary

• Rename and reorder sections
• Add captions to the toctree
• Move the python recipe to the python section
• Merge index and index_TOC for more idiomatic sphinx
• Update logo and css to match otb main website

Rationale

Part of #1679 and #1854 (closed)

Copyright

The copyright owner is CNES and has signed the ORFEO ToolBox Contributor License Agreement.

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Check before merging:

• All discussions are resolved
• At least 2 votes from core developers, no vote.
• The feature branch is (reasonably) up-to-date with the base branch
• Dashboard is green
• Copyright owner has signed the ORFEO ToolBox Contributor License Agreement

added documentation label

github pages is down right now, but in a few hours the output html should be available here: https://vpoughon.github.io/otb_cookbook_examples/

merged

mentioned in commit 78c3be1e

changed milestone to %7.0.0