added documentation and README file to linked list folder, minor fixes to README

[kseniavelichko]

Fixed broken Primorial link in the README, fixed minor style discrepancies in the README, added a README file to the linked list folder, and added documentation to the doubly linked list file in the form of a short description and minor comments explaining the code.

• [x] Read the contribution guidelines
• [x] Added unit tests (or unit tests have already been added)
• [x] Updated the Contents

Closes #455

[stale[bot]]

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[faheel]

@kseniavelichko Thanks for improving the documentation! Once the above comments are resolved this is good to be merged.