Docs/edc extensions readmes (#2711)

* Readme for DCP-extension

Signed-off-by: Zead-Alshu22 <zead.alshukairi@planen-bauen40.de>

* Readme for bdrs-client-extension

Signed-off-by: Zead-Alshu22 <zead.alshukairi@planen-bauen40.de>

* Readme for connector discovery-extension

Signed-off-by: Zead-Alshu22 <zead.alshukairi@planen-bauen40.de>

* Readme for agreements extension

Signed-off-by: Zead-Alshu22 <zead.alshukairi@planen-bauen40.de>

* chore: central extensions readme

Signed-off-by: Zead-Alshu22 <zead.alshukairi@planen-bauen40.de>

* overview README deleted

Signed-off-by: Zead-Alshu22 <zead.alshukairi@planen-bauen40.de>

* high-level module intent

Signed-off-by: Zead-Alshu22 <zead.alshukairi@planen-bauen40.de>

* revert agreements-Readme

Signed-off-by: Zead-Alshu22 <zead.alshukairi@planen-bauen40.de>

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Signed-off-by: Zead-Alshu22 <zead.alshukairi@planen-bauen40.de>

Author: Zead-Alshu22

edc-extensions/bdrs-client/README.md

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+## Purpose
+The BPN-DID Resolution Service (BDRS) Client extension provides a client for the BDRS server, which maintains mappings between Business Partner Numbers (BPNs) and Decentralized Identifiers (DIDs) in the Catena-X dataspace.
+The client maintains a local cache of these mappings for efficient resolution.
+For dataspaces that don't require mapping between a did and a different identifier, this extension (and the extensions injecting it) is obsolete.

edc-extensions/connector-discovery/README.md

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+## Purpose
+The Connector Discovery extension enables participants to discover other connectors in the dataspace.
+The **connector-discovery-api** module provides a REST API and default implementation for **DID-based** discovery with **Dataspace Protocol (DSP) 2025-1**. 
+It resolves connector endpoints from a participant’s DID document.

edc-extensions/dcp/README.md

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+## Purpose
+The DCP extension provides implementations for the Decentralized Credential Protocol.
+It handles credential scope extraction (from policies), Secure Token Service (STS) integration, and verifiable presentation caching.