doc: add table of contents to landing page

[coryan]

The landing page is easier to navigate (and to reference) if we have a
table of contents. I also fixed the nesting level for the different
sections.

---

This change is 

[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: 9924d330d4aae9ccd7f10a75c60b4d4ce80fadf1

• Public Access Build Logs
• Cloud Build Console (requires auth)
• Kokoro Logs (requires auth)

ℹ️ NOTE: Kokoro logs are linked from "Details" below.

[codecov]

Codecov Report

Merging #9671 (9924d33) into main (ace7f81) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #9671   +/-   ##
=======================================
  Coverage   94.33%   94.33%           
=======================================
  Files        1494     1494           
  Lines      138952   138952           
=======================================
+ Hits       131083   131085    +2     
+ Misses       7869     7867    -2     

Impacted Files	Coverage Δ
generator/internal/scaffold_generator.cc	83.59% <ø> (ø)
...cloud/pubsub/internal/subscription_session_test.cc	97.98% <0.00%> (-0.17%)	⬇️
google/cloud/pubsub/samples/samples.cc	90.77% <0.00%> (+0.07%)	⬆️
...ud/spanner/integration_tests/client_stress_test.cc	84.86% <0.00%> (+0.65%)	⬆️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	100.00% <0.00%> (+2.94%)	⬆️

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