MAINT: Update software stack

[mmcky]

This PR upgrades to:

• anaconda==2023.09
• python=3.11
• quantecon-book-theme==0.6.0

[mmcky]

@HumphreyYang do you have time to fix the link checker?

[HumphreyYang]

Hi @mmcky,

I saw there there was a notice for our old site, but the site has been taken down. Therefore, I have removed this part from the intro entirely.

Please let me know if we want to keep it there.

[mmcky]

thanks @HumphreyYang