minor: simplify docs build process & pin pip package versions

[Jefffrey]

• Pin pip packages and introduce dependabot config to auto-bump them for us - I believe there's been an overall strategy to try pin our dependencies to specific versions to mitigate supply chain risks, so this is following suit
• Update docs README a bit, add uv instruction to make it easier to build
• Remove need for temp dir as we now run the rustdocs_trim as part of the Sphinx process as an extension itself, and also remove the sed for relative links as there are no relative links anymore
  • Reference extensions documentation for Sphinx: https://www.sphinx-doc.org/en/master/extdev/index.html#dev-extensions

[Jefffrey]

We only ever did this to one file and it looks like the file doesn't have anymore relative URLs

[Jefffrey]

Here we register rustdoc_trim so it'll run during Sphinx build, meaning we don't need to manually fix it in source files that are in a temp dir, allowing us to use original build dir for simplicity.

Tested by checking the catalog page, can see it's properly trimmed:

Source code:

datafusion/docs/source/library-user-guide/catalogs.md

Lines 56 to 68 in a451920

	`tables` is the key-value pair described above. The underlying state could also be another data structure or other storage mechanism such as a file or transactional database.
	Then we implement the `SchemaProvider` trait for `MemorySchemaProvider`.
	```rust
	# use std::sync::Arc;
	# use dashmap::DashMap;
	# use datafusion::catalog::TableProvider;
	#
	# #[derive(Debug)]
	# pub struct MemorySchemaProvider {
	# tables: DashMap<String, Arc<dyn TableProvider>>,
	# }

[alamb]

I also double checked and it looks good to me

[alamb]

Looks beautiful to me -- thank you @Jefffrey

[alamb]

I also double checked and it looks good to me

[alamb]

nice!