Deployment instructions - make secret/variable formatting more clear

[annasoracco]

Please update all secrets and variables to include the simple rule/requirement to creating secrets and variables that will pass GH checks -

1. Format e.g. lower case only - or - Upper + Lower + number)
2. Min/max number of characters
3. Spacing or no spacing
4. etc.

[mithunshanbhag]

Will fix.

[mithunshanbhag]

All GH secret values have already been updated.

Now updated the documentation as well in private branch. Will merge after code-review: https://github.com/microsoft/contosotraders-cloudtesting/commit/2698e64cdfdedb26ed1287cb37f1cf43fddcbfec