Add a custom error page

[gaurab-khanal]

Currently the doc website shows Nextjs default error page for any error. It would be better if there is custom page for error.

[gaurab-khanal]

/help-wanted

[gaurab-khanal]

cc @naman9271 @oksaumya

[rahulshendre]

I can take this up. Please let me know if there are any preferences for the error page design or content.