docs: update api-reference project for v2

[shahednasser]

• Make v2 the default version of api-reference
• Set base path of V2 API reference to /v2. For example, /v2/api/admin.
• Remove all versioning-related snippets and functionalities except for the version switcher
• Change version switcher to only take the user to v1 API reference
• Move rewrites to book project, making it the main routing project for paths starting with /v2.
• Normalize environment variable names to match across projects.

Note: renaming specs-v2 directory to specs makes the number of file changes in this PR big, as that deletes v1 specs

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[shahednasser]

@olivermrbl can we get this one in?