Update Templates Readme - Github Repo links (paritytech#5381)

When someone downloads the Polkadot SDK repo and navigates to the
templates folder, the Readme instructions do not work. There is a
getting started script of the Polkadot SDK readme which can be
overlooked (and also it covers only minimal template and not the
parachain/solochain templates).

The instructions of the Readme files are updated such that they work for
anyone on
https://github.com/paritytech/polkadot-sdk
https://github.com/paritytech/polkadot-sdk-minimal-template
https://github.com/paritytech/polkadot-sdk-parachain-template
https://github.com/paritytech/polkadot-sdk-solochain-template

---------

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
Co-authored-by: Bastian Köcher <git@kchr.de>

templates/minimal/README.md

@@ -37,6 +37,14 @@ A Polkadot SDK based project such as this one consists of:
 * 🛠️ Depending on your operating system and Rust version, there might be additional
 packages required to compile this template - please take note of the Rust compiler output.
 
+Fetch minimal template code:
+
+```sh
+git clone https://github.com/paritytech/polkadot-sdk-minimal-template.git minimal-template
+
+cd minimal-template
+```
+
 ### Build
 
 🔨 Use the following command to build the node without launching it:

templates/parachain/README.md

@@ -39,6 +39,14 @@ A Polkadot SDK based project such as this one consists of:
 * 🛠️ Depending on your operating system and Rust version, there might be additional
 packages required to compile this template - please take note of the Rust compiler output.
 
+Fetch parachain template code:
+
+```sh
+git clone https://github.com/paritytech/polkadot-sdk-parachain-template.git parachain-template
+
+cd parachain-template
+```
+
 ### Build
 
 🔨 Use the following command to build the node without launching it:

templates/solochain/README.md

@@ -23,9 +23,17 @@ packages required to compile this template. Check the
 the most common dependencies. Alternatively, you can use one of the [alternative
 installation](#alternatives-installations) options.
 
+Fetch solochain template code:
+
+```sh
+git clone https://github.com/paritytech/polkadot-sdk-solochain-template.git solochain-template
+
+cd solochain-template
+```
+
 ### Build
 
-Use the following command to build the node without launching it:
+🔨 Use the following command to build the node without launching it:
 
 ```sh
 cargo build --release