[SYNTH-12978] Make improvements to the README (#1372)

* (DoD for ticket)make clear it's the same global config for all commands

This is the the commit for the 1/2 of the actions required for my ticket.
I'm explicitly calling out in the upload-application command that it can be the same Global Config file used for both commands.
Also renamed all the uses of a global config file i could see in the README to all use the same name for the file so it's less confusing when reading

* (DoD for ticket)improve the example global config file

Added all poossible configurations with example values

* add back the global.config.json as it's used in the build

* Addressing comments

* Arrange the setup otions to match the order of priority and also the next sections

* Update global config descriptions and reorder

* Update DATADOG_SYNTHETICS_LOCATIONS to correct name

* minor text fixes and reorderings

* refactoring list of configurations

* some tidying up and rearranging

* complete refactor of list of configurations

* shorten the proxy example as all of those configurations are not needed

* remove list of env variables as they are now shown in the List of Configurations

* move the list of configurations to into the run-tests section

* do the same for upload-application command as run-tests

* move Use Proxy to the end of the Setup

* moving the information about subdomain and locations to somewhere more relevant

* make the wording in the Setup more general for both steps

* do the same for Test files as i did for the other lists of configurations

* add Configuration options  title everywhere

* move upload-application command after run-tests command

* some formating

* move the bit about startUrl out of the first opening paragraph

* improve the information for the 3 ways we can configure

* add some info about using it as a library (might need further changes)

* update the Test files section

* when any test detects a regression, no?

* improve the using datadog-ci as a library section

* address Antoine's comments

* rewordings and spelling changes addressing comments

* explictly call the meanings of Global and Test Config

* oops forgot one boolean for the --no-...

* update example for using as a library

* move list of overrides to new Test overrides section and make configuration options bold

* move and rename the example global config file

* reference the Test oveirrides section in Test files section

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Co-authored-by: aliciascott <[email protected]>