doc: Improve RAK docs

[i3mSuperMan]

Summary

Improve documentation of RAK7240 guide, as The Things Stack supports TLS server authentication and client token authentication mode. Whereas the Connect RAK7240 in Basic Station Mode documentation mentioned with a TLS server authentication.
Ref: https://www.thethingsindustries.com/docs/gateways/rak-7240/bs/

Changes

• Changed the authentication mode to TLS server authentication and client token
• Added the RAK gateway screenshot for TLS server authentication and client token configuration
• Added the how to configure the CUPS key and LNS key files

This guide will help you set up the RAK2470 gateway to communicate over The Things Stack.

• Modified Gateway Model from RAK2470 to RAK7240 in the above sentence.

Note: Configuring and connecting RAK7249, RAK7268 and RAK7258 gateways to The Things Stack should be identical as for the RAK7240.

• Added the RAK7268 to the above Note point as the configuration for the gateway is identical to the RAK7240

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Run Locally: Verified that the docs build using make server, posted screenshots, verified external links. Test with HUGO_PARAMS_SEARCH_ENABLED=true if style changes will affect the search bar.
• New Features Marked: Documentation for new features is marked using the new-in-version shortcode, according to the guidelines in CONTRIBUTING.
• Style Guidelines: Documentation obeys style guidelines in CONTRIBUTING.
• Commits: Commit messages follow guidelines in CONTRIBUTING, there are no fixup commits left.