Add readme and some docs

[arturo-seijas]

Applicable spec:

Overview

Add readme, how to contribute and external access docs

Rationale

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 is generated using src-docs
• The documentation for charmhub is updated
• The PR is tagged with appropriate label (urgent, trivial, complex)

[erinecon]

Thanks for adding documentation for this charm!

One question I am wondering about is whether it's worth adding an index.md page for Charmhub into this PR. I don't know how much extra work or effort that will be for you.

[erinecon]

Thanks for the updates! A couple more comments and then should be good to go :)

[github-actions]

Test coverage for 0b93f29

Name                     Stmts   Miss Branch BrPart  Cover   Missing
--------------------------------------------------------------------
reactive/smtp_relay.py     285      0     90      0   100%
--------------------------------------------------------------------
TOTAL                      285      0     90      0   100%

Static code analysis report

Run started:2025-01-10 08:14:33.582176

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 2087
  Total lines skipped (#nosec): 11
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):

[erinecon]

Thanks for contributing to the docs!