Update source.md - Default to stable branch

[coincashew]

Motivation

Safer: To make the default git cloning behavior for "build from source" production safe, thus avoiding any misunderstandings.
Common default branch: All other consensus clients github repos default to their respective stable branch.

Description

Adds stable branch flag to git clone operation.

[CLAassistant]

All committers have signed the CLA.

[nflaig]

@coincashew Thanks for updating this but we can not merge changes directly to stable branch as those need to be included to unstable first and then will be merged from there.

We actually did a update to the source installation docs a while ago but for it to update the actual docs page it needs to be in stable but it currently just is in unstable, see PR #5056.

I opted for adding a note instead of directly specifying which branch should be cloned as I think if you run in prod it might even be best to specify a tag aka version instead of a branch.

[dapplion]

@nflaig users don't read notes they just copy commands (me included!) I think adding the stable branch as a safe default is good and additive to your note

[nflaig]

just need to adapt the note then as it currently says that unstable will be pulled by default, we have a bit of a conflict here since the note was added on unstable branch whereas this PR targets the stable branch

[nflaig]

Common default branch: All other consensus clients github repos default to their respective stable branch.

@coincashew looks like prysm does not do this either, they default to develop, in their docs they reference the master branch though. In your guide it points to master as well so should be fine

[nflaig]

@coincashew This is definitely a good update to the docs and I think as @dapplion mentioned we should just add the -b stable by default. Need to reopen this against the unstable branch and do the docs updates there to avoid merge conflicts when merging unstable to stable.

The base branch was changed.

[nflaig]

we also need to update the warning or just remove it entirely

lodestar/docs/install/source.md

Lines 30 to 32 in e98d8e9

	!!! warning
	`git clone` will check out the default `unstable` branch. If you are running Lodestar in production, we recommend to either use the `stable` branch
	by running `git switch stable` or to use a specific version by running `git checkout <version>`, e.g. `git checkout v1.3.0`.

[coincashew]

Requested change made. Good feedback all around. Thanks.

[nflaig]

lgtm, thanks for the update @coincashew

[wemeetagain]

🎉 This PR is included in v1.7.0 🎉