Port docs about "Contributing" from the wiki

### Task
Port the pages from the [Contributing](https://github.com/GlowstoneMC/Glowstone/wiki#contributing) section in the Glowstone wiki.

### Details
I would put these pages under the `Development` tree section. The link to the `Contributor License Agreement` can be put in a new page, which contains a link to the CLA inside.

### Guide
[Contributing to Docs](https://docs.glowstone.net/en/latest/Development/docs.html)

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