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The architecture and roadmap document in the doc/ folder seemed grossly outdated and of little value, so I removed them.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