Create covax-demo-app.md

[f-odhiambo]

Added a Markdown file with documentation for the Covax demo app as requested by the WHO team

IMPORTANT: Where possible all PRs must be linked to a Github issue

Fixes #1212

Checklist

• I have written Unit tests for any new feature(s) and edge cases for bug fixes
• I have added any strings visible on UI components to the strings.xml file
• I have updated the CHANGELOG.md file for any notable changes to the codebase
• I have run ./gradlew spotlessApply and ./gradlew spotlessCheck to check my code follows the project's style guide
• I have built and run the fhircore app to verify my change fixes the issue and/or does not break the app

[codecov]

Codecov Report

Merging #1225 (3ed7811) into main (5d69569) will increase coverage by 1.31%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##               main    #1225      +/-   ##
============================================
+ Coverage     73.33%   74.65%   +1.31%     
- Complexity      872     1317     +445     
============================================
  Files           155      226      +71     
  Lines          5520     8533    +3013     
  Branches        907     1384     +477     
============================================
+ Hits           4048     6370    +2322     
- Misses          835     1169     +334     
- Partials        637      994     +357     

Flag	Coverage Δ
anc	77.13% <ø> (?)
eir	66.75% <ø> (ø)
engine	74.68% <ø> (-0.06%)	⬇️
quest	73.20% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...data/remote/shared/interceptor/OAuthInterceptor.kt	68.75% <0.00%> (-12.50%)	⬇️
...anc/ui/anccare/encounters/EncounterListActivity.kt	77.77% <0.00%> (ø)
...ister/fhircore/anc/data/report/model/ReportItem.kt	27.27% <0.00%> (ø)
...fhircore/anc/ui/details/adapter/CarePlanAdapter.kt	100.00% <0.00%> (ø)
...hircore/anc/ui/anccare/details/EncounterAdapter.kt	100.00% <0.00%> (ø)
...rtregister/fhircore/anc/ui/report/ReportScreens.kt	79.58% <0.00%> (ø)
...c/ui/anccare/register/components/AncPatientList.kt	13.33% <0.00%> (ø)
.../anc/ui/family/form/FamilyQuestionnaireActivity.kt	61.70% <0.00%> (ø)
...nc/ui/family/register/components/FamilyListItem.kt	0.00% <0.00%> (ø)
...tregister/fhircore/anc/ui/login/AncLoginService.kt	87.50% <0.00%> (ø)
... and 62 more

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