I want to know is it possible to convert solochain to a relaychain so that parachain can connect with the chain but I was not able to see any tutorial, i came across this tutorial which converts a solochian to a parachain which will connect to a relaychain, is there a similar type of document that describes how to convert solochain to a relaychain.
In the tutorial you posted the term
solochain refers actually to a regular node based off the node template, and the guide allows you to "convert" it to an actual parachain by adding Cumulus to the node's codebase, so it can communicate with the relay chain. If you want to run a relay chain, you need to use the Polkadot node. This repo has the details how to run a local two node relay chain, for example. To this relay chain you can connect your parachain, which you can create from your
solochain. Hope this provides a bit more clarity. You can read more about Polkadot architecture here.
You can launch a relay chain testnet from the polkadot repo (as Lana mentioned), and i would recommend launching a Rococo chain. There are substrate docs to launch a local relay chain. That is the most practical way.
If instead of local relay chain you want to create a Live staging relay chain, so that other validators can connect to it, then you would need to change the chain spec to "Live" instead of Local and then add some initial validator keys.
To answer the question:
is it possible to convert a solo chain (substrate node-template) to a relay chain?
The answer is yes you can, but it is not practical to do so. You will need to add the necessary pallets and perhaps some changes in the node outside of the runtime. There is a learning adventure required in order to really understand what is going on there though.
I started a community run relay chain test net called Pop-Art, which is a Rococo fork. But i havent yet tried to convert a substrate node template to a relay chain, and makes its not practical there are not much docs on that workflow yet.
A great step is to add a validator to an already live relay network, before creating a relay network. Follow this tutorial how to add validators to a relay chain (and here is a similar issue to gain more breadcrumb insights.