Add redeployment and external-access docs

[weiiwang01]

Add two new documents: the redeployment guide and the external access Requirements.

The redeployment guide describes the steps users need to follow to migrate from an existing WordPress deployment to a new charm deployment. The external access requirements document details the necessary internet access for WordPress to function properly after deployment.

[yanksyoon]

LGTM!

[github-actions]

Test coverage for 92b7682

Name                Stmts   Miss Branch BrPart  Cover   Missing
---------------------------------------------------------------
src/charm.py          540     35    184     33    91%   162, 203-206, 370, 429->433, 582, 613, 619, 636-640, 670, 705-706, 764, 773, 877->890, 889, 891, 896-897, 957, 977, 984, 1074, 1083, 1095, 1116, 1125, 1144, 1148, 1177, 1230, 1362, 1384, 1391->1393, 1436->exit, 1448, 1458, 1496
src/cos.py             21      0      0      0   100%
src/exceptions.py      17      1      2      1    89%   41
src/state.py           34      0      4      0   100%
src/types_.py          16      0      0      0   100%
---------------------------------------------------------------
TOTAL                 628     36    190     34    91%

Static code analysis report

Run started:2024-05-02 17:39:40.477498

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 4586
  Total lines skipped (#nosec): 1
  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):