fix(docs): DSP-API docs root route gives a 404 error (DEV-3345)

[BalduinLandolt]

Pull Request Checklist

Task Description/Number

For https://docs.dasch.swiss/latest/DSP-API/ (in our combined docs setup - in an isolated setup, this corresponds to the / route) to be available, MkDocs needs a top level index.md file.

Issue Number: DEV-3345

PR Type

• build/chore: maintenance tasks (no production code change)
• docs: documentation changes (no production code change)
• feat: represents new features
• fix: represents bug fixes
• perf: performance improvements
• refactor: represents production code refactoring
• test: adding or refactoring tests (no production code change)

Basic requirements for bug fixes and features

• Tests for the changes have been added
• Docs have been added / updated

Does this PR introduce a breaking change?

• Yes

Does this PR change client-test-data?

• Yes

[linear]

DEV-3345 Fix 404 on https://docs.dasch.swiss/latest/DSP-API/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.66%. Comparing base (eac5751) to head (89cd26a).
Report is 91 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3073       +/-   ##
===========================================
+ Coverage   11.74%   88.66%   +76.92%     
===========================================
  Files         246      262       +16     
  Lines       22907    22480      -427     
===========================================
+ Hits         2690    19932    +17242     
+ Misses      20217     2548    -17669     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[seakayone]

suggestion (non-blocking): Add a small table of contents to the overview

I feel like the current first page https://docs.dasch.swiss/latest/DSP-API/01-introduction/what-is-dsp/ gives a good overview/advertisement of the DSP platform.
Now that we have an actual "Overview" for just the api documentation it might be useful to point to some interesting starting point to dig deeper into the documentation.

Could be a bullet point list which is adding some information to the most interesting pages.
Something like:

Here are some interesting pages to get started:
* DSP Ontologies - An in depth description of data modelling
* DSP-API Endpoints - A user guide on how to interact with our platform with your own clients
* FAQ - Answers to common inquiries presented concisely for quick reference

[seakayone]

praise: Thanks for swiftly solving this problem.