Fix links, typos, and errors

[ellenfieldn]

This pull request includes a handful of documentation updates to improve clarity, correct references, and fix minor errors across multiple files. The most important changes are summarized below:

Corrections and improvements to documentation:

• docs/guides/basic-usage.md: Updated the project fork link to point to the correct repository.
• docs/guides/intro.md: Fixed the link to the Ptyxis terminal to point to the correct URL because the prior URL was broken.
• docs/guides/software.md:
  • Updated Flatpak's URL because the prior URL was not working due to: SSL certificate with incorrect domain used for https://www.flatpak.org URL flatpak/flatpak.github.io#697 [1]
  • Swapped an absolute URL to relative URL.. [2]
• Fixed a handful of grammatical errors.
• Updated default icon documentation from colorful to white. It appears that the white start icon is enabled by default in Aurora as of earlier this week.