Merge common steps from ESMValTool release documentation to ESMValCore

[ehogan]

Compare the content of https://docs.esmvaltool.org/en/latest/community/release_strategy/release_strategy.html#how-to-make-an-esmvaltool-release and ESMValCore .

Remove ESMValTool content and merge with ESMValCore.

Identify links with "ESMValTool" and "ESMValCore", use Sphinx tabs for more complicated differences, if necessary.

[jlenh]

Tentative overview:

• Step 1 "Check that all tests and builds work" is similar but links differ depending on ESMValTool/ESMValCore
• Step 2 should be "Increase the version number", needs formatting update for setuptools-scm link, content slightly differs for ESMValTool as ESMValCore version number needs matching
• Step 3 should be "Add release notes", text slightly differs at the end, but the information should be the same.
• Step 4 should be "Create a release branch", text slightly differs but the information should be the same.
• Step 5 "Make the (pre-)release on GitHub", potential difference between ESMValCore that might need several pre-releases and ESMValTool that might need none. More information should be added about how to tag the release branch from GitHub or from the command line. Mention to add a link to the current changelog as for previous releases.
• Step 6 "Mark the release in the main branch" should be removed as it is not needed anymore when working only on the release branch.
• Step 7 "Create and upload the PyPI package" should be removed as it is not needed anymore because it is automatically uploaded to PyPI when creating a pre-release or a release on GitHub.
• Step 8 "Create the Conda package" might slightly differ between ESMValTool and ESMValCore so the content could be partly kept the same and partly separate, notably for the pre-release of ESMValCore.
• Step 9 "Create the Conda package" might need to be updated, but the information should be the same.