[Debt] Rules in `_styles_project.scss` should be shifted to their respective files

### Current Behavior
[`assets/scss/_styles_project.scss`](https://github.com/layer5io/docs/blob/929a84e14840010fee135c1135f10396ecb9de8f/assets/scss/_styles_project.scss) has been bloated by the addition of lots of rules that should ideally be in other files. Take [`.td-sidebar`](https://github.com/layer5io/docs/blob/929a84e14840010fee135c1135f10396ecb9de8f/assets/scss/_styles_project.scss#L189) for example: this rule really belongs in [_sidebar-tree.scss](https://github.com/layer5io/docs/blob/929a84e14840010fee135c1135f10396ecb9de8f/assets/scss/_sidebar-tree.scss). There are [some rules in other files](https://github.com/layer5io/docs/blob/929a84e14840010fee135c1135f10396ecb9de8f/assets/scss/_subscription.scss#L27) that belong to the wrong file.

### Expected Behavior
Splitting `_styles_project.scss` would prevent having to deal with a bloated monolith down the line. Other out of place rules should be identified as well to improve the codebase.

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#### Contributor Guide and Resources
- 📚 [Instructions for contributing to documentation](https://github.com/layer5io/docs/blob/master/CONTRIBUTING.md)
- Layer5 documentation [site](https://docs.layer5.io) and [source](https://github.com/layer5io/docs/)
- 🎨 Wireframes and [designs for Layer5 site](https://www.figma.com/file/5ZwEkSJwUPitURD59YHMEN/Layer5-Designs) in Figma [(open invite)](https://www.figma.com/team_invite/redeem/GvB8SudhEOoq3JOvoLaoMs)
- 🙋🏾🙋🏼 Questions: [Layer5 Discussion Forum](https://discuss.layer5.io) and [Layer5 Community Slack](http://slack.layer5.io)

Start by reading assets/scss/_styles_project.scss and compare its selectors with the partials, especially _sidebar-tree.scss and _subscription.scss. Inventory rules that clearly belong elsewhere, move them to the appropriate existing files, and verify the documentation site keeps the same styling while _styles_project.scss is no longer a bloated monolith.

Written by the indexing model from the issue text.