Fix ReadTheDocs configuration

[fabcor-maxiv]

Use the --force flag of mamba env create in case environment exists.

Skip poetry run when running sphinx-build since Poetry installs in the conda environment directly.

Remove the workaround for the unwarranted error about "missing conda.environment" configuration setting, because it has been fixed already. See:

• https://github.com/readthedocs/readthedocs.org/pull/10979#issuecomment-1896036953
• https://github.com/readthedocs/readthedocs.org/pull/11040

[fabcor-maxiv]

For some reason that I can not pinpoint yet, the ReadTheDocs builds complain that the conda environment already exists, like in this one for example: https://readthedocs.org/projects/mxcubecore/builds/23347508/

I hope using the --force flag will smooth things out.

[fabcor-maxiv]

I will probably need to do the same for mxcubeweb.

[fabcor-maxiv]

In this case I will merge myself before external review, because:

• it seems straightforward to me
• it is kind of blocking other pull requests I want to make
• I am more or less the only one who knows this stuff