fix: document pushing files with colon in name

[TerryHowe]

What this PR does / why we need it:

Document procedure to push a file with colon in the file name.

Fixes https://github.com/oras-project/oras-www/issues/134

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.05%. Comparing base (00a19d2) to head (9840e8a). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1508 +/- ## ======================================= Coverage 86.05% 86.05% ======================================= Files 118 118 Lines 4223 4225 +2 ======================================= + Hits 3634 3636 +2 Misses 351 351 Partials 238 238 ```

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[TerryHowe]