Move any non-Docker disk creation flows from README to separate docs

[letmaik]

I suggest to remove the following subsections in the "### 2. Creating SGX-LKL disk images with sgx-lkl-disk" section:

• "#### Creating Alpine-based disk images"
• "#### Creating plain disk images"

I also think the following sections can be removed:

• "### 3. Running applications from the Alpine Linux repository"
• "### 4. OpenJDK Java Virtual Machine (JVM)"
• "### 5. Cross-compiling applications for SGX-LKL"

The result is something along the lines of "use docker" and look at the samples for specific workloads (like Java).

EDIT: Instead of removing completely, the non-Docker parts may go into separate docs, with the main README only having a simple Getting Started sample.

[prp]

I agree that Java and cross-compilation should go, and instead become sample applications. I still think that there is value in supporting a particular distro (Alpine) directly.

[letmaik]

That doesn't scale. Either we offer generic mechanisms (e.g. Docker), or we have to add support for other distributions as well, like --ubuntu in the future, but I think that's a bad idea.

[prp]

Right, I would only keep it for the moment when Alpine is the only supported distribution. Once that limitations has disappeared, I agree that it should be removed.

[letmaik]

I would then (re)move the following:

• "#### Creating plain disk images"
• "### 3. Running applications from the Alpine Linux repository"
• "### 4. OpenJDK Java Virtual Machine (JVM)"
• "### 5. Cross-compiling applications for SGX-LKL"

Note that 3. is outdated and not working anyway, and I think superseded by Docker since that involves a more production type disk setup.