No more pre-building by-default, switching to recursive builds

[devanshshukla99]

❌ stalled until branches can be selected via conf.py

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The package, currently, first prebuilds all branches and tags via vanilla sphinx in an isolated location and then builds all the specified branches/tags via the sphinx-versioned-docs extension.
This may work for small projects, however, as the number of branches increases along with the project size, a more reasonable approach will be to build all branches with sphinx-versioned-docs and if any fails then initiate the building workflow for all branches except the failed ones.

This recursive approach will improve build times.
Branches to be ignored can be specified via --branch arg as mentioned in #69
 
(If any branch build fails then the building workflow is re-initated for all branches except the failed one. Pre-building can still be enabled via --prebuild cli arg.)

• Build tests passed
• Codestyle tests passed
• Added test coverage for new features
• Documentation updated to reflect the changes

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