Little improvement in the regtest manual

[Iximiel]

Description

@Puncist suggested that the manual page about the regtest could be clearer if the list of the types would be a nested list instead of a paragraph, and I agree.

Now developer-doc/html/regtests.html should appear like this:

I think this can be easily backported in any older version and then cherry-picked into the modern ones.

Target release

I would like my code to appear in release 2.9~11

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.