Delete internals pages

[zuiderkwast]

Some pages about the internals of Valkey (actually Redis), all of them at least a decade old and outdated, incorrect and don't belong in docs. These are not docs for users. (Information for developers doesn't belong in the docs.)

• Virtual Memory, a feature deleted decades ago
• SDS, a string library used in Valkey, but the internals have changed so the doc isn't right
• Event library, a page structured like a blog post, somewhat useful for develpers but not for users.
• Redis design draft 2, the RDB version 7 format (2013). It's not docs. It doesn't belong here.
• Internals overview page. Not useful, now that we're deleting all of these pages.

Besides, these pages were rebranded to Valkey but they're actually describing very old versions of Redis, so the text lost its context.

Fixes #109
Fixes #110
Fixes #129