[PyFDB] docs: Markdown fix and reference to FDB docs

[tbkr]

Description

Modified readme.md for pyfdb wheel.

Removed sphinx artifacts and added reference to FDB documentation.

Contributor Declaration

By opening this pull request, I affirm the following:

• All authors agree to the Contributor License Agreement.
• The code follows the project's coding standards.
• I have performed self-review and added comments where needed.
• I have added or updated tests to verify that my changes are effective and functional.
• I have run all existing tests and confirmed they pass.

🌈🌦️📖🚧 Documentation FDB 🚧📖🌦️🌈
https://sites.ecmwf.int/docs/dev-section/fdb/pull-requests/PR-262

[Copilot]

Pull request overview

Updates the PyFDB wheel README to remove Sphinx/reStructuredText artifacts and point users toward the upstream FDB documentation/resources.

Changes:

• Converted FDB references from reStructuredText link syntax to Markdown.
• Reordered and rewrote the “Documentation” section to reference FDB project pages and upstream repo guidance.
• Normalized command snippet formatting in the installation section.

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[Copilot]

The Markdown link target github.com/ecmwf/fdb is missing a URL scheme, so GitHub will treat it as a relative link. Use an absolute URL (e.g., https://github.com/ecmwf/fdb).

[Copilot]

This Markdown link also uses github.com/ecmwf/fdb without a scheme, which will render as a relative link. Update to https://github.com/ecmwf/fdb.

[Copilot]

There is an unmatched opening backtick before the link text, which will break Markdown rendering for the rest of the line/section. Remove the stray backtick (or add the matching closing backtick) so the link renders correctly.

Suggested change

	For implementation details and tooling, see the `[FDB project pages](https://fields-database.readthedocs.io/en/latest/index.html).
	For implementation details and tooling, see the [FDB project pages](https://fields-database.readthedocs.io/en/latest/index.html).

[Copilot]

This link target is missing a URL scheme (github.com/ecmwf/fdb), which makes it a relative link in Markdown. Use https://github.com/ecmwf/fdb instead.

Suggested change

	To build the latest documentation locally, follow the guide at [FDB](github.com/ecmwf/fdb).
	To build the latest documentation locally, follow the guide at [FDB](https://github.com/ecmwf/fdb).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.19%. Comparing base (db7d78a) to head (440353a).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #262   +/-   ##
========================================
  Coverage    71.19%   71.19%           
========================================
  Files          376      376           
  Lines        23762    23762           
  Branches      2478     2478           
========================================
  Hits         16918    16918           
  Misses        6844     6844           

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