Fix broken links in sample repos with custom default branch

[zubron]

Closes #9157

Change Description

Background

The generated README.md for sample repositories hardcoded the default branch as main. This meant that if a user specified a custom default branch name, the image asset links would be broken and tutorial commands would be incorrect.

Bug Fix

This change introduces a new config field for the templater to use the specified default branch and updates the README.md template to use this new field. The existing screenshots still use main as the default branch so various documentation lines and image alt-texts have been updated to reflect this.

Testing Details

I verified this fix by running lakeFS locally with a repository using a custom default branch:

1. Created a test repository (test-custom-branch) with custom-branch as the default branch name (instead of main)
2. Generated the sample README.md for this repository
3. Verified that all references to the branch name were correctly updated throughout the README

Screenshots of the fix

Repository configuration showing custom default branch name:

README intro correctly references custom-branch:

The note states that the tutorial's screenshot images still show main.

CLI examples use the correct branch name:

Query examples use the correct branch name:

[itaigilo]

Great stuff @zubron , congrats for your first lakeFS PR!

Basically this LGTM,
But blocking with two meta-comments:

1. Please add testing details. This well might be something like "I ran lakeFS locally, created a new repo with a non-main default branch, and verified that 'main' doesn't appear in the created README". Or providing a screenshot of such README with a different default branch name.
2. Note that a include/exclude-changelog label is required. I personally think it's changelog worthy, but I'm not sure.

Anyway, looking good!

[zubron]

@itaigilo Thanks for the review! You're right, I should have included those details. I've updated the description with how I tested it locally, and added a selection of screenshots to show how the correct branch name is being used and that the images are correctly linked. I agree on including it in the changelog as it is fixing a user facing bug, so I've added that label.

Hopefully that addresses all your concerns, but please let me know if there's anything else I can do to improve it :)

[itaigilo]

LGTM 😎

Congrats -
Feel free to squash-and-merge.