Migrate p2_index wiki pages to mark-down

[vogella]

For https://github.com/eclipse-equinox/p2/issues/497

[github-actions[bot]]

Test Results

    6 files  ±0      6 suites  ±0   19m 40s :stopwatch: -12s 2 184 tests ±0  2 180 :white_check_mark: ±0  4 :zzz: ±0  0 :x: ±0  4 426 runs  ±0  4 419 :white_check_mark: ±0  7 :zzz: ±0  0 :x: ±0 

Results for commit fcd5fb1e. ± Comparison against base commit 8bcba76d.

:recycle: This comment has been updated with latest results.

[vogella]

Mac failure seems unrelated

[merks]

Thank you. 👍