doc/radosgw: Cosmetic and formatting improvements in vault.rst

doc/radosgw: Cosmetic and formatting improvements in vault.rst

Use generated "contents" instead of repeating section titles in an
ordered list.

Use Title Case for section titles.

Capitalize consistently "Ceph" and "Object Gateway", "Vault".

Use consistently all caps for acronyms.

Change one entry of "kms" to "SSE-KMS", seems obvious from the context.
Use "certificates" instead of "certs".

Use double backticks for file paths, commands, config options etc.

Improve language in one sentence, several words missing.

Call it consistently "the Object Gateway" and not sometimes "the
Gateway".

Add pre-formatted block for CLI command examples where missing (should
be promptified in the future).

Signed-off-by: Ville Ojamo <14869000+bluikko@users.noreply.github.com>