[ext-pages] Fix for specifying an initial page before sending the paginator

[krittick]

Summary

This sets the initial page in Paginator.respond() and Paginator.send() to Paginator.current_page instead of the current value of 0. Since current_page defaults to 0, this should not be a breaking change.

This allows users to set the initial page before sending the paginator. For example:

@slash_command()
async def pagetest_initial(self, ctx: discord.ApplicationContext):
    """Demonstrates setting an initial page before sending."""
    paginator = pages.Paginator(pages=["Page 1", "Page 2", "Page 3", "Page 4"])
    paginator.current_page = 2
    await paginator.respond(ctx.interaction)

Fixes #858

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• If type: ignore comments were used, a comment is also left explaining why
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, typehinting, examples, ...)

[bensaine]

This looks good to me!