doc: update index.rst to include references to scion-application-docs

[JordiSubira]

After an offline discussion from SCION contributors, we reached the conclusion that docs.scion.org is the best-suited candidate to host technical information about SCION applications (ideally already in a certain maturity level) since they can be run on different environments, i.e., production network, SCIONLab or local dev environment. It may also help to centralize a little bit the information and making the documentation environment a bit less hairy for the users/developers.

In this PR, I ported the Applications section in the SCIERA docs , adding the file doc/applications/access.rst as a preliminary documentation as how to connect the application host to the diverse SCION networks.

We can revisit, if some of documentation for the currently listed applications must be removed or updated.

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We use RTD subprojects and the project is located in https://github.com/scionproto-contrib/scion-applications-docs.

This PR adds reference to the SCION Applications subproject.

[jiceatscion]

This change is 

[jiceatscion]

Hmm, I'm not sure if we should do that. These applications are not part of the scionproto repo. This means that their maintainers and the scionrpto repo maintainers aren't necessarily the same people. Would the scion TC implementation want to become a necessary agent for every change made to these applications, which requires updating their documentation?

I understand the desire to make everything about scion easy to find, but I am not so sure this is the way to go.

Other opinions?

[oncilla]

Have you considered using RTD subprojects: https://docs.readthedocs.io/en/stable/subprojects.html for this?

Reviewable status: 0 of 7 files reviewed, all discussions resolved (waiting on @jiceatscion)

[JordiSubira]

@jiceatscion it is a fair point, I also thought about that and at the beginning I also thought about a dedicated readthedocs for SCION applications. However, I also believe it is a valid concern having to many places for the information, especially right now. At this moment, the people administrating this will be the same people as attend the SCION contributors meetings. In the future, hopefully many new developers comes into the picture, but in the near/mid-term I do not see much of a problem with us curating this, because it will be mostly us or people supervised by us, the ones who will contribute to this.

@oncilla I will take a look into this, definitely, it seems like a good solution that would suite both cases, i.e. having everything centralized while dividing maintenance/curation efforts. Thank you :)

[JordiSubira]

Following @oncilla suggestion, we moved the contents to a subproject https://docs.scion.org/projects/scion-applications/en/latest
I will update the descritpiton accordingly

[jiceatscion]

Reviewed 1 of 1 files at r3, all commit messages.
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[jiceatscion]

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[jiceatscion]

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Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @JordiSubira)