docs: add landing pages

[florentianayuwono]

Applicable spec:

Overview

Add how-to and reference landing pages.

Rationale

To aid user in navigation.

Juju Events Changes

Module Changes

Library Changes

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, senior-review-required, documentation)
• The docs/changelog.md is updated with user-relevant changes in the format of keep a changelog v1.1.0
• The application version is incremented in app/pyproject.toml

[yanksyoon]

LGTM after minor linting fixes!

[erinecon]

The landing pages look good to me -- thank you so much for adding them!

The linkchecker is complaining about a link in a separate document, so no need to act on it unless you'd like to fix it in this PR. But I think the updated link is https://pipx.pypa.io/stable/how-to/install-pipx/

[florentianayuwono]

thanks @erinecon i have updated the link as well!