feat: Add .txt file support and root-level content in navbar

[LamPham123]

Purpose of this pull request:

Adds .txt file support
Root-level content files (ex. content/Welcome.txt) appear in the nav bar for both .txt and .md files.

IMPORTANT - UI CHANGE DEMONSTRATION

Screenshots:

Site preview link: https://lampham123.github.io/

Other things worth discussing regarding PR:

• If users create .txt .md and folders with the same name, that configuration error is handled with a popup telling the users to rename the conflicting files
• Markdown and Text are root-level content files from the demo website -> content/Markdown.md, content/Text.txt