AXIS2-6107 Fix broken anchor links, remove clustering docs, update Axis2/C URL

Site anchor fixes:
- http2-integration-guide.xml: "HTTP/2" → "HTTP2" in heading to prevent
  slash encoding (.2F) in generated anchor ID
- json-streaming-formatter.xml: "Problem:" → "Problem -" in section name
  to prevent colon encoding (.3A) in generated anchor ID
- toc.xml: 14 broken # anchor links corrected to match actual generated
  IDs (markdown .md files produce heavily-encoded anchors with .28/.29
  for parens and .E2.80.94 for em-dashes)
- toc.xml: openapi-rest-userguide anchors updated (#Samples →
  #Sample_Services_Overview, #Swagger_UI → #Interactive_Swagger_UI)
- toc.xml: spring-boot-starter #containers → #Container_matrix

Clustering removal (section 17 already removed from site):
- toc.xml: clustering-guide.html link removed
- contents.xml.vm: clustering-guide.html link removed

Axis2/C site URL:
- index.xml: http://axis.apache.org/axis2/c/ →
  https://axis.apache.org/axis2/c/core/ (HTTPS + correct path)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>