Document home add title

[Yang09701194]

Overview: Describe the external change and the impact/benefit from a user perspective.

Document home page html <title> is empty, I find it's defined in template base.html in package mkdocs-material, after open issue in https://github.com/squidfunk/mkdocs-material/issues/5661 , htmltitle can easily override and add the functionality as a customization ,

in setup-and-theme-structure, openj9-docs/mkdocs.yml set custom_dir: theme, so I suggest add main.html in theme folder as this commit, I test local it would add title <title>OpenJ9</title> to home.

[Sreekala-Gopakumar]

Thanks for the suggestion. We could look into this when we update mkdocs next time.