Added functionality to add DOIs to syntax file and then cite them using plumed.cite command

[gtribello]

Description

This is a continuation of PR #1199. This is just the stuff for adding DOIs to the syntax to json file so they can appear in the new manual and the stuff so that you can do plumed.cite().

Target release

I would like my code to appear in release 2.11

Type of contribution

• changes to code or doc authored by PLUMED developers, or additions of code in the core or within the default modules
• changes to a module not authored by you
• new module contribution or edit of a module authored by you

Copyright

• I agree to transfer the copyright of the code I have written to the PLUMED developers or to the author of the code I am modifying.

• the module I added or modified contains a COPYRIGHT file with the correct license information. Code should be released under an open source license. I also used the command cd src && ./header.sh mymodulename in order to make sure the headers of the module are correct.

Tests

• I added a new regtest or modified an existing regtest to validate my changes.
• I verified that all regtests are passed successfully on GitHub Actions.

[gtribello]

Hi @GiovanniBussi

We discussed this branch this morning @carlocamilloni is happy enough to merge this into master (with the changes I just made) if you are, too.

If you can merge if you are happy, that would be great.