feat(sdk): Change masking character to a bullet

[DRK3]

Also made some improvements to the Resolve Display docs.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (1d1d531) 89.95% compared to head (e6945a8) 89.95%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #579 +/- ## ======================================= Coverage 89.95% 89.95% ======================================= Files 98 98 Lines 3882 3882 ======================================= Hits 3492 3492 Misses 258 258 Partials 132 132 ``` | [Files Changed](https://app.codecov.io/gh/trustbloc/wallet-sdk/pull/579?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trustbloc) | Coverage Δ | | |---|---|---| | [pkg/credentialschema/credentialdisplay.go](https://app.codecov.io/gh/trustbloc/wallet-sdk/pull/579?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=trustbloc#diff-cGtnL2NyZWRlbnRpYWxzY2hlbWEvY3JlZGVudGlhbGRpc3BsYXkuZ28=) | `98.66% <100.00%> (ø)` | |

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[sonarcloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
3.0% Duplication