Sidebar restructure and minor clean-up

[pavithraes]

This PR:

• Organizes notebooks into "Tutorials" and "Example applications" dropdown sections
• Creates a sub-section for "Introduction to data loaders" within "Tutorials"
• Update several page titles for consistency and with more topic information -- for instance, removing phrases like "using JAX and Flax" where it seemed redundant
• Makes minor updates to some page text and links
• Updates contributing guide (closes minor refresh to dev docs needed #132)

I did look into intra-site cross-links. While we can add more cross-links to pages like the data-loaders in several notebooks under example applications, there are already links to specific Grain data loaders and such, which seemed enough. I can include more where relevant in follow-up PRs related to the sidenotes below.

Link to preview: https://jax-ai-stack--136.org.readthedocs.build/en/136/tutorials.html

---

Sidenotes:

I noticed a few more areas to update but did not include here because it was creeping up the scope:

• The TensorBoard tutorial page could benefit from narration updates and organizing into sections. I started on it as a part of this PR, but can open a separate scoped-PR.
• It might be worth updating for consistent:
  • Language and style: Consistent grammatical person ("you" > "we"), voice (active), etc.
  • Page structure: Like having consistent sections for setup and installation, further reading/summary, etc. across most pages. (This is not required for every single page. For example, the getting started page has a good flow, but several others have the similar sections defined differently.)
    Didn't want to include these updates here because some decisions are subjective. Again, I can open a separate scoped-PR for this.
• The "Introduction to Data Loaders" pages for CPU and GPU are very similar (I believe Jake also noted this earlier). I created a separate sidebar dropdown for now, but we may want to consolidate this into one page with specific notes for different hardware, or have one primary content and document only differences for different hardware in the specific pages.

[jakevdp]

This looks really great, thanks! Two small comments below.

[jakevdp]

Looks really great – thanks for working on this! I also love the ideas you have for further clarification & organization.

Thanks!