Add docs to metadata for gatekeeper

[Thanhphan1147]

Gatekeeper is using the main branch to fetch the charmhub url to the charm's doc set. This cause CI in PRs to fail because no PR has been merged to update this field

Applicable spec:

Overview

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)

[github-actions]

license-eye has totally checked 34 files.

Valid	Invalid	Ignored	Fixed
0	10	24	0

Click to see the invalid file list

• charmcraft.yaml
• config.yaml
• generate-src-docs.sh
• metadata.yaml
• src/charm.py
• tests/conftest.py
• tests/integration/init.py
• tests/integration/test_charm.py
• tests/unit/init.py
• tests/unit/test_base.py

[github-actions]

Test coverage for db787e0

Name           Stmts   Miss Branch BrPart  Cover   Missing
----------------------------------------------------------
src/charm.py      30      0      4      0   100%
----------------------------------------------------------
TOTAL             30      0      4      0   100%

Static code analysis report

Run started:2024-01-08 17:32:21.457768

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 149
  Total lines skipped (#nosec): 0
  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):