Rewrite README as a lean, capability-organized quickstart (#197)

## What

Rewrites the package README as a lean, capability-organized quickstart
(~240 lines, down from ~1150). Microsoft Learn and `examples/` remain
the source of truth for exhaustive depth; the README is the agent- and
PyPI-facing quickstart.

## Why

The README is the PyPI page and the doc surface coding agents keep
in-context. The long comprehensive README duplicated Learn (and had
drifted from it) and buried the high-value paths. This trims it to a
fast, accurate quickstart and routes depth to Learn.

## Key changes

- **Usage organized by capability** -- Create/read/update/delete, Query
records, Define and evolve schema, Work in bulk, Async client, Handle
errors -- ordered by how often each is reached for (queries and metadata
reads dominate production usage).
- **Concept taught inline + contextual pointers** -- each section
explains its concept where the code shows it, then deep-links the
matching Learn article and runnable sample.
- **Removed duplication** -- dropped the flat Learn link list that
mirrored the inline pointers (kept only orientation links: Overview /
Quick guide / Getting started); the intro no longer re-lists Key
features; Key features tightened to scannable fragments; the async
section no longer re-instructs the `[async]` install.
- **Verified against source** -- single `create` returns a GUID `str`;
`retrieve` returns `None` on 404; `tables.create` takes a typed-column
dict; `create_lookup_field`; the error hierarchy has 5 classes including
`SQLParseError` (the live error-handling Learn page lists only 3 -- the
README follows the code).
- **PyPI-safe links** -- all repo links are absolute `https://`;
ASCII-only; no relative links.
- **Single client construction** -- shown once, as a context manager
(Authenticate builds only the credential).
- Contributing / CLA / Trademarks preserved verbatim from `main` (only
the `operations/` link was made absolute for PyPI).

## Reviewer action

- Confirm the capability coverage and ordering match how you want the
SDK presented.
- **Run the six Usage code blocks against a live org** (CRUD, Query,
Schema, Bulk, Async, Errors) to confirm they execute as written before
merge.
- Decide on the two `API Design Guidelines` points that assume the old
comprehensive-README model ("add a README example per public method" and
"keep README and SKILL in sync") -- left verbatim; they may want
revisiting under the lean-README approach.

## Verification done

- All links checked: repo paths exist on `main`; the Learn / Microsoft
URLs resolve to the correct topic and match the section that links them;
PyPI page and badges resolve.
- Exception import validated against `src/.../core/errors.py` (all 5
classes exported).
- ASCII-clean, balanced code fences, no leftover template placeholders.
- Mandatory Contributing / CLA / Trademarks byte-identical to
`origin/main`.

Note: the "preview" banner on the current PyPI page is a stale-snapshot
artifact from the 1.0.0 publish; it is already removed on `main`, and
publishing 1.0.1 clears it. Out of scope here.

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Co-authored-by: Abel Milash <abelmilash@microsoft.com>

Author: suyask-msft