Remove `sphinxcontrib-bibtex` as a documentation-dependency

[danielhuppmann]

Description of PR

~This PR updates the docs to work with the latest sphinxcontrib-bibtex version for compatibility with Python 3.10.~

This PR

• removes using bibtex (and the package sphinxcontrib-bibtex as a dependency) because of strange formatting output on ReadTheDocs.
• refactors the folder structure to be compatible with the latest ReadTheDocs configuration (conf.py & index.rst have to be at the same location), see https://github.com/readthedocs/readthedocs.org/issues/7883#issuecomment-799376427

[codecov[bot]]

Codecov Report

Merging #735 (4486e01) into main (e07d3b9) will increase coverage by 0.0%. The diff coverage is 91.4%.

@@          Coverage Diff          @@
##            main    #735   +/-   ##
=====================================
  Coverage   95.0%   95.1%           
=====================================
  Files         59      59           
  Lines       6020    6044   +24     
=====================================
+ Hits        5725    5748   +23     
- Misses       295     296    +1     

Impacted Files	Coverage Δ
pyam/utils.py	92.7% <91.1%> (+0.1%)	:arrow_up:
tests/test_tutorials.py	94.5% <100.0%> (ø)
tests/test_core.py	100.0% <0.0%> (ø)

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[danielhuppmann]

Harominjzing the build-the-docs-test, the nightly (weekly) test, and the RTD-configuration in anticipation of dropping Python 3.7 support (which is the current RTD default).

Test-build on https://pyam-iamc.readthedocs.io/en/docs-sphinxcontrib-bibtex/