.gitignore: refactor, unify & update

[ia]

• Please check if the PR fulfills these requirements

• The changes have been tested locally
• There are no breaking changes

• What kind of change does this PR introduce?
  Since .gitignore is presented & maintained for this repo, probably it should be updated due to some recent changes & due to actualization of the content:

• refactor to make common style
• toss already added lines around file to the more related sections
• replace ci/artefacts by scripts/ci/artefacts due to recent root directory refactoring
• add site/ directory for locally generated static docs by MkDocs
• add ignore for files of source code tagging systems like GNU Global, ctags, and cscope

• What is the current behavior?
  Current behavior of git may not be reliable for such files and/or directorys as site/, scripts/ci/artefacts, etc. Plus, since .gitignore mentions all main IDEs and their configuration files, adding popular source code tag files may be reasonable as well.

• What is the new behavior (if this is a feature change)?
  With new behavior git should ignore explicitly such files as site/, scripts/ci/artefacts, tags, GTAGS, cscope, etc.

• Other information:
  P.S. I guess once a Pull-Request is merged & closed, a user may not receive notifications for new comments. If so, I would like to ask to look at this comment which may be not seen yet (and the one just above). Thanks :)

[Ralim]

I've seen your other message, just haven't been at PC to do it.

I get an email for everything that happens on here 😂