Mention the esmvaltool-base package in the installation instructions

[bouweandela]

Description

Mention the new esmvaltool-base package in the documentation. Installation of this package with minimal dependencies, requires just 2 GB of disk space, while the full esmvaltool package needs 6 GB and the previous smallest option esmvaltool-python required about 4 GB.

• Related to Installing ESMValTool and dependencies - a look at the future #3997
• Link to documentation: https://esmvaltool--4595.org.readthedocs.build/en/4595/quickstart/installation.html

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