Zoekt: Enable internalApi by default

What does this MR do?

This MR enables Zoekt internal API by default as well as updates the documentation. We're doing that to prepare for the SM Zoekt Alpha launch. This reduces the effort to set up Zoekt.

Related issues

Related to gitlab-org/gitlab#438493 (closed)

Testing

git@charts-toolbox-7d75d44588-htgtl:/$ gitlab-rails console -e production
--------------------------------------------------------------------------------
 Ruby:         ruby 3.1.4p223 (2023-03-30 revision 957bb7cb81) [x86_64-linux]
 GitLab:       16.9.0-pre (08ce5c4693a) EE
 GitLab Shell: 14.32.0
 PostgreSQL:   14.8
------------------------------------------------------------[ booted in 54.12s ]
Loading production environment (Rails 7.0.8)
irb(main):001:0> ::Feature.enable(:index_code_with_zoekt)
irb(main):001:0> ::Feature.enable(:index_code_with_zoekt)
WARNING: Understand the stability and security risks of enabling in-development features with feature flags.
See https://docs.gitlab.com/ee/administration/feature_flags.html#risks-when-enabling-features-still-in-development for more information.
=> true
irb(main):002:0> ::Feature.enable(:search_code_with_zoekt)
irb(main):002:0> ::Feature.enable(:search_code_with_zoekt)
WARNING: Understand the stability and security risks of enabling in-development features with feature flags.
See https://docs.gitlab.com/ee/administration/feature_flags.html#risks-when-enabling-features-still-in-development for more information.
=> true
irb(main):003:0> node = ::Search::Zoekt::Node.last
=>
#<Search::Zoekt::Node:0x00007fd1b35b3448
...
irb(main):004:0> namespace = Namespace.find_by_full_path('zoekt-test')
=> #<Group id:2 @zoekt-test>
irb(main):005:0> enabled_namespace = Search::Zoekt::EnabledNamespace.find_or_create_by(namespace: namespace)
=>
#<Search::Zoekt::EnabledNamespace:0x00007fd1abfc49a8
...
irb(main):006:0> node.indices.create!(zoekt_enabled_namespace_id: enabled_namespace.id, namespace_id: namespace.id, state: :ready)
=>
#<Search::Zoekt::Index:0x00007fd1abbc85c0
 id: 1,
 zoekt_enabled_namespace_id: 1,
 zoekt_node_id: 2,
 namespace_id: 2,
 created_at: Fri, 19 Jan 2024 17:26:54.147859146 UTC +00:00,
 updated_at: Fri, 19 Jan 2024 17:26:54.147859146 UTC +00:00,
 state: "ready">
irb(main):007:0>

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