fix: remove self managed client api

[zoedsoupe]

Problem

After deprecating the self-managed client pattern in #80, the README and module documentation still extensively
documented and promoted the Agent-based pattern. This could confuse new users who might implement the deprecated
(and unsafe) pattern instead of the recommended functional approach.

Solution

• Removed references to "two ways to create clients" (one-off vs self-managed)
• Simplified client initialization documentation to focus on the single recommended pattern
• Removed @moduledoc sections in lib/supabase/client.ex that explained the Agent-based usage
• Updated lib/supabase.ex docstrings to remove mentions of self-managed clients

Rationale

Documentation should reflect the recommended patterns, not deprecated ones. Since the self-managed pattern is now
deprecated (#80) due to race condition vulnerabilities, keeping it prominently featured in docs would actively
harm users by encouraging unsafe implementations.

The simplified docs now show only the functional pattern—creating lightweight client structs on-demand—which is
both safer and more idiomatic for server-side Elixir applications. Users still on the old pattern will see the
deprecation warnings when they compile.

Note: This PR depends on #80 (deprecation) being merged first.

Release-As: 0.8.0

[zoedsoupe]

the will generate a breaking change

[Ziinc]

Needs minor version bump