Update DOI URLs

[e-kwsm]

Summary

Include a summary of major changes in bullet points:

• URLs are changed to https://doi.org/ in accordance with https://www.doi.org/doi_handbook/2_Numbering.html#htmlencoding.

Checklist

Work-in-progress pull requests are encouraged, but please put [WIP]
in the pull request title.

Before a pull request can be merged, the following items must be checked:

• Code is in the standard Python style. The easiest way to handle this
  is to run the following in the correct sequence on your local machine. Start with running
  black on your new code. This will automatically reformat
  your code to PEP8 conventions and removes most issues. Then run
  pycodestyle, followed by
  flake8.
• Docstrings have been added in the Google docstring format.
  Run pydocstyle on your code.
• Type annotations are highly encouraged. Run mypy to type check your code.
• Tests have been added for any new functionality or bug fixes.
• All linting and tests pass.

Note that the CI system will run all the above checks. But it will be much more efficient if you already fix most
errors prior to submitting the PR. It is highly recommended that you use the pre-commit hook provided in the pymatgen
repository. Simply cp pre-commit .git/hooks and a check will be run prior to allowing commits.

[mkhorton]

Thank you @e-kwsm!

[coveralls]

Coverage decreased (-0.6%) to 83.112% when pulling d7044eb on e-kwsm:doi into 9276567 on materialsproject:master.

[mkhorton]

@e-kwsm, since you are a new contributor, please do fill out this form so we can acknowledge you appropriately in pymatgen documentation