chore(stellar-wallet-snap): add Stellar snap docs + update gitignore

[stanleyyconsensys]

Explanation

This PR does

• Update stellar docs where it was git ignored during migration
• Remove packages/*/docs ignore from the git ignore

References

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them