Restructuring developer documentation for easier reach

[subhramit]

contributing.md used to be an independent topic in the devdocs in itself.
Apart from having its own original content as index, we propose to make it a parent that has two children:

1. The "Getting into the code" section. This contains:
   1a."JabRef's development strategy",
   1b."Setting up a local workspace" and
   1c."High-level documentation"
   This used to be an independent section in itself, which will be shifted here.
2. Developer FAQs (Refer Add FAQ to devdocs (Code Howtos) #11456).

This makes important documentation more prominent to reach for newcomers.

This PR also fixes some grammatical errors in FAQs, and adds some pointers about reaching IntlliJ Settings in M2 Macs while setting up the development environment.

Update: Also adds the link to the developer FAQs in the github actions bot newcomer greeting message.

Mandatory checks

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[koppor]

Try to avoid negations: please check our instead of don't forget to...

[subhramit]

Moving to WIP till:

• Links pointing to the moved parent topics are fixed
• Screenshots are added in PR Description

[subhramit]

We will probably have to close this because: https://just-the-docs.com/docs/navigation-structure/#example-children-within-children

Currently, the navigation structure is limited to 3 levels: grandchild pages cannot themselves have child pages.

[koppor]

Will be huge effort to finalize. Thus, better improve the Markdown files (as you did). We will do a larger restructuring at another time.