[Docs] Improve contributing docs (apache#1965)

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Closes apache#1964

# Rationale for this change
Following all steps in the contributing docs leads to error. Running
`make install-poetry` should be done before the environment is
activated.

# Are these changes tested?
Yes

# Are there any user-facing changes?
No
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Author: petern48

mkdocs/docs/contributing.md

@@ -37,18 +37,10 @@ The PyIceberg Project is hosted on GitHub at <https://github.com/apache/iceberg-
 For the development, Poetry is used for packing and dependency management. You can install this using:
 
 ```bash
-pip install poetry
+make install-poetry
 ```
 
-Make sure you're using an up-to-date environment from venv
-
-```bash
-pip install --upgrade virtualenv pip
-python -m venv ./venv
-source ./venv/bin/activate
-```
-
-To get started, you can run `make install`, which installs Poetry and all the dependencies of the Iceberg library. This also installs the development dependencies. If you don't want to install the development dependencies, you need to install using `poetry install --no-dev`.
+To get started, you can run `make install`, which installs all the dependencies of the Iceberg library. This also installs the development dependencies. If you don't want to install the development dependencies, you need to install using `poetry install --without dev` instead of `make install`.
 
 If you want to install the library on the host, you can simply run `pip3 install -e .`. If you wish to use a virtual environment, you can run `poetry shell`. Poetry will open up a virtual environment with all the dependencies set.