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

[iulianbarbu]

More details posted here: https://github.com/paritytech/polkadot-sdk/pull/5954/files/8b4884dc8a9d60d09d61b08ada0e645121c0d6b8#r1825695159. The idea is to keep parachain-template README.md in sync with the hardcoded configuration set in the Rust code.

[seemantaggarwal]

Had a word with @iulianbarbu about this. We will divide this into three parts:

1. Creating a docify template to generate a Readme in parachain template
2. mirror the current state of Readme.md through the docify tempalte
3. Replace the hardcoded value during compile time
4. Test by changing the value, not a commit

We will track each step through this PR only

[seemantaggarwal]

Important example to look at
substrate/bin/utils/chain-spec-builder

This already uses a docify, I will study and understand how the template works here, and then use the knowledge gained to mirror it for the parachain template

[iulianbarbu]

Following up on a topic I discussed previously with @seemantaggarwal , and putting my finding for reference here: I recall why we added the dollar sign ($) for $runtime_path example I showed you from chain-spec-builder.

We also run tests for the export_content sections that can be found in chain-spec-builder crate (so that we get testable documentation which is kept up to date given these tests run in CI). The $ is needed at runtime not by docify but by cmd_lib crate (used to execute bash like commands in the rust tests). It interpolates variables that are in scope into the bash-like commands: e.g. https://docs.rs/cmd_lib/latest/cmd_lib/#macros-to-run-external-commands. We shouldn't need a similar test for the scope of this issue, only testing the exported const value changes in the readme manually after it is modified in the code.