[feat] Convert the documentation to Read The Docs template

[dhunstack]

Following changes were made in this commit -

• Sphinx's conf.py file was regenerated. Old configuration file is in conf_old.py
• custom.css was added to set sidebar colors
• documentation.rst page was added to serve as the home page for docs. Replaces the old documentation.html.
• Some rst files were modified to reduce the number of sidebar pages

[dbogdanov]

@dhunstack The build-docs workflows is missing sphinx_rtd_theme dependency (Theme error: no theme named 'sphinx_rtd_theme' found (missing theme.toml?)), it needs to be added here.

[dbogdanov]

Fixed doc build script to catch such issues in the future: https://github.com/MTG/essentia/commit/c896c90b452849a1acc5ff13c622bf9e128ad8a1.

[dhunstack]

PR for dependency updates - https://github.com/MTG/essentia/pull/1444 Can merge that in master first to avoid creating another merge commit here.