Merge full-text search, security, concepts, ML commands pages

[Dhghomon]

This next PR does the following:

• Merges the full-text reference guide into the data models / full-text search page. Adds new content here and there such as the ability to use the FUNCTION clause (not many people know about it). The reference guide was originally for both FTS and working with text, so have modified the title a bit to "Text and full-text search" to match.
• Merges the concepts page into a single one (i.e. removes the separate pages for namespace and database).
• Merges the ML commands into a single page as there are only two.
• Moves the security best practices page into the Security section so that it will have better visibility.
• Reorders the querying section because GraphQL should not come before SurrealQL.
• Updates the output of the CLI commands.

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[welpie21]

Looking good!