docs: More structured migration guide

[sentrivana]

• added subsections for integrations, api changes, etc.
• added a small span streaming migration guide

[github-actions]

Codecov Results 📊

✅ 55226 passed | ❌ 2 failed | ⏭️ 2727 skipped | Total: 57955 | Pass Rate: 95.29% | Execution Time: 150m 24s

📊 Comparison with Base Branch

Metric	Change
Total Tests	📉 -79751
Passed Tests	📉 -75358
Failed Tests	📈 +2
Skipped Tests	📉 -4395

➕ New Tests (1)

View new tests

• test_cache_spans_decorator
  • File: tests.integrations.django.test_cache_module
  • Status: ❌ Failing

❌ Failed Tests

test_cache_spans_decorator

File: tests.integrations.django.test_cache_module
Suite: py3.13-django-v6.1.1
Error: tests/integrations/django/test_cache_module.py:307: in test_cache_spans_decorator assert not spans[0]["attributes"]["cache.hit"] E assert not True

Stack Trace

tests/integrations/django/test_cache_module.py:307: in test_cache_spans_decorator
    assert not spans[0]["attributes"]["cache.hit"]
E   assert not True

✅ Patch coverage is 100.00%. Project has 2061 uncovered lines.
❌ Project coverage is 90.03%. Comparing base (0a75283) to head (ce097c8).

Coverage diff

@@            Coverage Diff             @@
##        master       #PR       +/-##
==========================================
- Coverage    90.37%    90.03%    -0.34%
==========================================
  Files          202       185       -17
  Lines        26547     20673     -5874
  Branches      9852      7114     -2738
==========================================
+ Hits         23990     18612     -5378
- Misses        2557      2061      -496
- Partials      1496      1215      -281

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