Use docify export for parachain template hardcoded configuration and embed it in its README #6333 (#7093)

Use docify export for parachain template hardcoded configuration and
embed it in its README #6333

Docify currently has a limitation of not being able to embed a
variable/const in its code, without embedding it's definition, even if
do something in a string like

"this is a sample string ${sample_variable}"

It will embed the entire string 
"this is a sample string ${sample_variable}"
without replacing the value of sample_variable from the code

Hence, the goal was just to make it obvious in the README where the
PARACHAIN_ID value is coming from, so a note has been added at the start
for the same, so whenever somebody is running these commands, they will
be aware about the value and replace accordingly.

To make it simpler, we added a 
rust ignore block so the user can just look it up in the readme itself
and does not have to scan through the runtime directory for the value.

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Co-authored-by: Iulian Barbu <14218860+iulianbarbu@users.noreply.github.com>