[docs]: Fix issues with Rust code snippets in "quick start"

[prrao87]

The renaming of vectordb to lancedb broke the quick start docs (it's pointing to a non-existent directory). This PR fixes the code snippets and the paths in the docs page.

Additionally, more fixes related to indexing docs below 👇🏽.

[prrao87]

Apologies for combining two tasks in this one PR, but I forgot to switch branches 😅. These second set of commits address the manual indexing requirements in the docs.

Indexing docs

To clarify to users why we need to index manually, I'd word it carefully in three places:

• Quick start
• Concepts
• Guides

Remedies

• In the quick start section, we add an admonition box stating why we need to index manually: first, that we don't always need an ANN index because LanceDB is 🔥 for datasets of <1M vectors, and second, because there's no one-size-fits-all when it comes to indexing (most other vector DB vendors have sane defaults, and HNSW is more forgiving, but that doesn't mean that those settings are optimal)
• In the "ANN indexes" section of the guides, we add a subsection titled "Why do I need to manually create an index?" clarifying the same things
• In the "Concepts" section covering the IVF_PQ index, I've removed the FAQ section (it's repetitive as it already appears twice in the FAQ and the Guides sections), but I link to the guides section as that's where the most detailed explanation on how/why we index sits.

All in all, I think this helps a new user gain a sequential understanding on how/why indexing is done in LanceDB. Because IVF_PQ is fundamentally different from HNSW, some manual tuning is desirable in most cases and it's best to let the user be aware of that through each of these sections. Because we cannot predict which section a user might chance upon when they first encounter LanceDB, it's best to drop these hints/explanations in all these sections.

@changhiskhan @AyushExel and @raghavdixit99, if you have more additions or suggestions on top of these, am happy to go through and offer my inputs. Hope this makes sense!

[prrao87]

@AyushExel No idea about why CI is failing here, seems like a lot has changed since my last PR!

[changhiskhan]

@prrao87 thanks! We've done some major refactoring so that we can support an Async API and a much better Rust API in general. I'm fixing the CI in a separate PR. Once that's merge I'll update this one so CI can pass.

[changhiskhan]

this lgtm. Just one small nit. Thanks!

cc @westonpace in case there's additional places where we need to update vectordb references to lancedb instead