docs: updated the contributing page due to additional discord bot set up

[Kenspectacle]

also improved some sentences clarity

[what-the-diff]

PR Summary

• Enhanced Environment Configuration Instructions
  The instructions for setting up the environment variables have been updated to make it easier for you to adjust the settings of the bot.

• New Installation Steps Section
  A new section detailing the necessary adjustments to specific settings during installation has been added. These include:

  • Switching off "User Install" and activating "Guild Install"
  • Setting "Install Link" to "None" and saving your changes

• Emphasis on Saving Changes
  A strong emphasis has been placed on saving changes post any adjustment to the settings, whether in the Installation tab or on the Bot page. This will ensure the effective implementation of all configured settings.

[vcarl]

Looks good, tyty!