Update device agent docs and link to from node-red

[allthedoll]

Description

Update device agent docs and link to from node-red:

• Add one line install instructions to the introduction page
• Add nick's gif of terminal to the docs image folder instead of linking to one for blog
• Add said gif to quickstart page
• Add it to the introduction page too why not

Unrelated:

• Fixed various typos and minor grammatically issues because I am who I am

Related Issue(s)

Checklist

• I have read the contribution guidelines
• Suitable unit/system level tests have been added and they pass
• Documentation has been updated
  • Upgrade instructions
  • Configuration details
  • Concepts
• Changes flowforge.yml?
  • Issue/PR raised on FlowFuse/helm to update ConfigMap Template
  • Issue/PR raised on FlowFuse/CloudProject to update values for Staging/Production
• Link to Changelog Entry PR, or note why one is not needed.

Labels

• Includes a DB migration? -> add the area:migration label