Fix README: broken docs-build command, broken link, typos, stale content

[animmosmith]

Summary

Fixes #409.

• "Build docs locally" was broken outright - verified by actually running it: the documented sphinx-apidoc -f -o docs/source docs/build --separate fails immediately since neither docs/source nor docs/build exist. Cross-checked .readthedocs.yaml (which builds via docs/conf.py directly) and confirmed the correct, working command is a single sphinx-build -b html docs/ docs/_build/html - no separate apidoc step needed, since sphinx.ext.autosummary (already configured in docs/conf.py) generates the API docs automatically.
• Fixed a broken markdown link ((uv)[url] → [uv](url)) and a Windows-style backslash path inside a bash command block.
• Fixed typos (regester, monatge, complie, awarenes, plesant, aknowledged) and inconsistent "PyOpia"/"PyOPIA" naming throughout.
• Replaced the "Development targets" section - stale, and in places directly contradicting the current architecture (e.g. it said "no use of settings/config files," but the whole Pipeline is config-driven) - with a short, accurate "Design principles" section.
• Clarified step 5's montage.png as an output file rather than a command, and noted that init-project --example-data both downloads images and generates config.toml, so it doesn't appear to come from nowhere.
• Expanded the Docker rationale beyond "for users who prefer not to install dependencies directly" (reproducibility, avoiding heavy ML dependency installs, consistent behaviour across OS, HPC/server use).

Test plan

• Ran the full quick-start flow end-to-end with real example data: init-project --example-data → process → merge-mfdata → make-montage (including the corrected forward-slash path) → confirmed montage.png is produced
• Ran the corrected "Build docs locally" command and confirmed it actually builds

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