Add RStudio instructions to configuration - fixes #497

[helijuottonen]

Fixes #497

Added

1. Mention of configuration via RStudio to the first section of configuration page and a link to the 'Authenticating to GitHub' on the same page.

Note: is the internal link to the section ''Authenticating to GitHub' here done correctly? Please check and change as needed.

2. RStudio tab to the last section 'Authenticating to GitHub' with

• link to instructions in the longer installations lesson
• instructions to use HTTPS when cloning
• R command for setting default branch to main (this might not be necessary?)
• mention that other configuring commands on the page can be run in the RStudio terminal tab (does this work or is it confusing?)

[bast]

Thank you!