sourcing static text on chat-page from env file

[wmgillett]

• Replaces hard-coded text (e.g. "Chat with your Legal Docs") with parameters sourced from the .env file.
• Allows for 4 existing text fields on the chat page to be customized without changes to code in the page/index.tsx file.
• Code leverages server-side rendering in Next.js framework and follows conventions in the existing code base.
• Simplifies most standard customizations by consolidating them into a single env file already in use.
• Number of fields in .env file increases from 4 to 10.
• Default values (based on current hard-coded values) are provided for all new parameters so use of the existing .env files (with only 4 parameters) will not generate errors when running new code.
• Minor updates to the README are recommended and can be provided if needed.
• Changes to the QA_PROMPT are not covered by this change.