docs: add start pages and workflow documentation

[AlexanderLanin]

• Add a user guide start page
• Add a reference section start page
• Add user guide -> typical workflow page
  I'm a little fuzzy on step 11 myself, as my changes are always stuck in approved state... 😉

[Copilot]

Pull request overview

This pull request adds documentation structure and content to improve navigation and provide workflow guidance for otterdog users. The changes introduce index pages for major documentation sections and a comprehensive workflow guide.

Changes:

• Added index pages for User Guide and Reference sections to improve documentation navigation
• Created a new workflow documentation page describing the typical pull request-based process for making configuration changes

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
mkdocs.yml	Added navigation entries for new index pages in User Guide and Reference sections
docs/userguide/workflow.md	New workflow guide documenting the 11-step process for making changes via pull requests
docs/userguide/index.md	New User Guide index page providing overview and links to tutorials and feature documentation
docs/reference/index.md	New Reference section index page providing organized links to documentation resources

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