chore: flatter exports, docs fixes and README header

[roeap]

What changes are proposed in this pull request?

Some small docs fixes. E.g. while docs.rs seems to render links fine when the reference is also wrapped backticks - [something]: ... - rust analyzer does not recognize this, and requires a "plain" name.

The architecture and roadmap document in the doc/ folder seemed grossly outdated and of little value, so I removed them.

Also wanted to propose some flair for the Readme :).

This PR affects the following public APIs

While there are no actual API changes, some structs can now be imported (IMO) more conveniently. Building out the datafusion integration, I found myself frequently having to write deep imports - e.g.

// before
use delta_kernel::engine::default::executor::tokio::TokioBackgroundExecutor;

// after
use delta_kernel::engine::default::TokioBackgroundExecutor;

How was this change tested?

current tests + rendering and inspecting docs with

cargo doc -p delta_kernel --all-features

[roeap]

interesting, docs build seems to fail now, while it did work locally... I'll see whats going on.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.45%. Comparing base (c3a868f) to head (7e5d374).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #609   +/-   ##
=======================================
  Coverage   83.45%   83.45%           
=======================================
  Files          74       74           
  Lines       16877    16877           
  Branches    16877    16877           
=======================================
  Hits        14084    14084           
  Misses       2135     2135           
  Partials      658      658           

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[scovich]

// comments are unceremoniously stripped out by the parser and disappear.
/// comments become annotations that cbindgen can actually see and work with.

I agree it's unfortunate that this pollutes the public docs, tho.

[nicklan]

thanks! just one thought about the html in the readme

[nicklan]

mmm, this does look nice rendered by github. I don't love dumping html into our readme though. also the align attribute is deprecated in html5, so maybe not the best to use in new stuff?

I guess I don't generally love assuming people will only view this on github in the browser. I'd like cat README.md to work okay, which maybe it still does, but certainly not quite as well.

thoughts?

If we gave up centering could the rest be done in "normal" markdown?