Add GitHub Actions and GitLab to supported build servers in docs

[gdziadkiewicz]

Description

Added two missing CIs with support in Fake.

TODO

Feel free to open the PR and ask for help

• New (API-)documentation for new features exist (Note: API-docs are enough, additional docs are in help/markdown)

• unit or integration test exists (or short reasoning why it doesn't make sense)

  Note: Consider using the CreateProcess API which can be tested more easily, see https://github.com/fsharp/FAKE/pull/2131/files#diff-4fb4a77e110fbbe8210205dfe022389b for an example (the changes in the DotNet.Testing.NUnit module)

• boy scout rule: "leave the code behind in a better state than you found it" (fix warnings, obsolete members or code-style in the places you worked in)

• (if new module) the module has been linked from the "side navigation" menu, edit docs/data.json.

• (if new module) the module is in the correct namespace.

• (if new module) the module is added to Fake.sln (dotnet sln Fake.sln add src/app/Fake.*/Fake.*.fsproj)

• Fake API guideline is honored

[github-actions]

Test Results

   12 files  ±0     12 suites  ±0   32m 28s ⏱️ -8s
  451 tests ±0    450 ✅ ±0  1 💤 ±0  0 ❌ ±0 
1 287 runs  ±0  1 284 ✅ ±0  3 💤 ±0  0 ❌ ±0 

Results for commit 27987b7. ± Comparison against base commit 74e1e6f.