Fix section structure error (6. Cross-domain Collaboration, 7. System Integration)

[k-toumura]

(related issue/PR: #628, #771)
I didn't notice in on the Arch call in May 24th, some structure of section seems broken.
There is extra </section> tag on after "5.9 Virtual Things":

wot-architecture/index.html

Lines 1568 to 1571 in 02b9aea

	</p>
	</section>
	</section>

This extra tag accidentally makes the subsequent "Cross-domain Collaboration" and "System Integration" subsection independent from the Section 5 "Common Deployment Patterns".

This PR remove the extra section tag, and also remove introduction text for "Cross-domain Collaboration" and "System Integration" (and also fix section ID). The resulting section structure is as follows:

  ...
5. Common Deployment Patterns
5.1 Telemetry
  ...
5.9 Virtual Things
5.10 Cross-domain Collaboration
5.11 System Integration
6. Abstract WoT System Architecture
  ...

(if the original section structure is intentional, please close this PR)

---

Preview | Diff

[mlagally]

Why did you remove that paragraph?

[k-toumura]

Because "Introduction" contains a description of sections, not subsections.
"Cross domain collaboration" and "System Integration" section are moved to subsections in Section 5
(5.10 and 5.11, respectively).