Version 0.8.0b1 Version bump, documentation, and warning

[marklysze]

Why are these changes needed?

Adds warnings on four pages noting that the openai package is no longer installed by default.

• Quick Start
• Basic Concepts > Installing AG2
• Basic Concepts > LLM Configuration > LLMs
• Model Providers > OpenAI

Related issue number

N/A

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/docs/contributor-guide/documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ There is a different number of reports uploaded between BASE (4b2c2dd) and HEAD (8a898c4). Click for more details.

HEAD has 1134 uploads less than BASE

Flag	BASE (4b2c2dd)	HEAD (8a898c4)
core-without-llm	14	1
3.9	67	0
ubuntu-latest	131	1
3.13	76	0
macos-latest	96	0
3.11	65	1
3.10	87	0
windows-latest	107	0
3.12	39	0
commsagent-discord	3	0
optional-deps	105	0
retrievechat	15	0
interop-crewai	9	0
retrievechat-pgvector	10	0
jupyter-executor	9	0
retrievechat-mongodb	10	0
interop-langchain	9	0
websockets	9	0
interop-pydantic-ai	9	0
docs	6	0
interop	13	0
graph-rag-falkor-db	6	0
twilio	9	0
retrievechat-qdrant	14	0
agent-eval	1	0
gpt-assistant-agent	3	0
mistral	14	0
groq	14	0
teachable	4	0
llama-index-agent	3	0
long-context	3	0
lmm	4	0
retrievechat-couchbase	3	0
swarm	14	0
cohere	15	0
bedrock	14	0
ollama	14	0
together	14	0
cerebras	14	0
gemini	15	0
reasoning	14	0
anthropic	16	0
websurfer	15	0
core-llm	3	0
deepseek	1	0
falkordb	2	0
autobuild	1	0
integration	10	0
crawl4ai	3	0
browser-use	3	0
neo4j	2	0

Files with missing lines	Coverage Δ
autogen/version.py	100.00% <100.00%> (ø)

... and 45 files with indirect coverage changes