docs: remove legacy docs folder

[Arun-Niranjan]

Addresses:

• Can we remove the docs folder? #408

and also:

• https://github.com/infer-actively/pymdp/security/dependabot/1
• https://github.com/infer-actively/pymdp/security/dependabot/2
• https://github.com/infer-actively/pymdp/security/dependabot/4
• https://github.com/infer-actively/pymdp/security/dependabot/5
• https://github.com/infer-actively/pymdp/security/dependabot/6

because we don't use jinja when generating docs the new way

[conorheins]

thanks for this, agree with the motivation in #408 and getting the 5 jinja2 dependabot alerts to disappear as a side effect is a nice bonus.

Two small things before we land:

1. Preserve three orphan notebooks. these have no counterpart anywhere in examples/:

• docs/notebooks/active_inference_from_scratch.ipynb
• docs/notebooks/pymdp_fundamentals.ipynb
• docs/notebooks/using_the_agent_class.ipynb

could you move them into examples/legacy/ before deleting the rest? i think they are useful educational material despite being outdated in terms of referencing numpy backend. that dir is already excluded from the notebook manifests so there won't be any CI overhead. the other three notebooks in docs/notebooks/ (cue_chaining_demo, tmaze_demo, free_energy_calculation) already have counterparts in examples/ so those are fine to drop.

2. stale line to delete. docs-mkdocs/development/viewing-docs.md:34 says `docs/` is retained as legacy Sphinx-era content for compatibility/history. which becomes false after this PR, so worth dropping in the same go.

once those tiny changes are in, think this PR is good to go

[conorheins]

great, lgtm!