rework of contribution guide section

[aaperis]

Describe the pull request:

• Documentation change

Pull request long description:

The text is partially rephrased at places, links were updated and a separation between contributing to the codebase repo and the handbook repo was made.

Additional information:

I noticed that there are some Issue templates here that seem somewhat obsolete. Do we keep them? If yes, I can add a mention to these cases as well.

[blankdots]

I noticed that there are some Issue templates here that seem somewhat obsolete. Do we keep them? If yes, I can add a mention to these cases as well.

those don't seem relevant now, except the new question.

also we might want to adjust the Pull request template to https://github.com/neicnordic/sensitive-data-archive/blob/main/.github/PULL_REQUEST_TEMPLATE.md as it is simpler

[blankdots]

small suggestion, otherwise looks pretty good.

I am fine including template changes in this PR or another one

[aaperis]

I noticed that there are some Issue templates here that seem somewhat obsolete. Do we keep them? If yes, I can add a mention to these cases as well.

those don't seem relevant now, except the new question.

also we might want to adjust the Pull request template to https://github.com/neicnordic/sensitive-data-archive/blob/main/.github/PULL_REQUEST_TEMPLATE.md as it is simpler

Opened #102 PR with changes regarding the github templates.