Add link to the single page documentation in the index [BATCH-2682]

[spring-projects-issues]

Mahmoud Ben Hassine opened BATCH-2682 and commented

Currently, the single page documentation file is correctly generated (https://docs.spring.io/spring-batch/4.0.x/reference/html/index-single.html) but there is no link to it. The link "reference" in the home page points to the multi page version and the user has to rename the url from "index.html" to "index-single.html" to get access to it.

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Affects: 4.0.0

Referenced from: pull request #582, and commits 67cf37d