docs: add Prerequisites block to memory guide (fixes #1077)

[somo9909]

Fixes #1077

What this PR does

Adds a Prerequisites block to docs/guides/memory.mdx
modelled after docs/guides/code.mdx (lines 55-90).

The block covers:

• Install path (developer clone)
• Extras required: [dev, rag]
• Lemonade server must be running
• Examples folder is inside the cloned repo

Notes

Starting with memory.mdx first for feedback before
updating the remaining 13 guides.

[somo9909]

Hi @kovtcharov! This is my first guide update for #1077.
I've added the Prerequisites block to memory.mdx following
the code.mdx template. Please let me know if any changes
are needed before I update the remaining 13 guides.

[kovtcharov]

@claude review this update to the guide. Should we include Gaia init and Agent UI for targeting different users when onboarding with memory, not just developers?

[github-actions]

Yes — the Prerequisites should cover end users too, because the guide's own body recommends the Agent UI as the primary way to enable memory (gaia chat --ui, the Memory Dashboard), yet the new block only describes the developer clone path. A reader who followed the recommended UI flow won't recognize themselves in these prereqs.

Quickstart already splits these audiences (docs/quickstart.mdx: Agent UI desktop app for end users → gaia init → Manual/Clone for developers), so mirroring that here keeps things consistent. A <Tabs> block — the same component quickstart uses — is the cleanest fit:

• Agent UI (end users): install the desktop app; it sets up the backend and bundles the RAG runtime automatically, so no manual extras. Enable memory from the Memory Dashboard / gaia chat --ui.
• Developers: the current clone + uv pip install -e ".[dev,rag]" block.

One accuracy note in your favor, @somo9909: the [rag] extra is genuinely required for the developer path — memory loads faiss + sentence-transformers + numpy directly (src/gaia/agents/base/memory.py). The [ui]/Agent UI install pulls those same deps in for you (#845), which is why the UI path needs no extras line.

Minor nit: the diff dropped the trailing newline at end of file — worth restoring.

Great first contribution — splitting the prereqs by audience before rolling out to the other 13 guides will set a good template.