docs: bump to Odra 2.7.2 and 2.8.0

[kpob]

Brings the docs up to date with the latest odra releases on crates.io (2.7.0/2.7.1 are superseded by 2.7.2; 2.5.1 intentionally skipped).

For each of 2.7.2 and 2.8.0:

• Version bump applied across docusaurus/docs/ (Cargo.toml snippets, docs.rs links, GitHub release links) — 14 files, 38 replacements per version
• versions.json updated (now 2.8.0, 2.7.2, 2.6.0, …)
• Snapshot created under versioned_docs/version-2.7.2/ and versioned_docs/version-2.8.0/ with matching sidebar files
• lastVersion in docusaurus.config.js set to 2.8.0

Manual-review notes:

• migrations/ left untouched (historical versions); there is no to-2.7.2.md/to-2.8.0.md migration guide — add one if either release warrants it.
• tutorials/using-proxy-caller.md: the [package] version of the odra-examples crate was bumped along with the release version, since it tracks Odra releases.
• Pre-existing typo (not changed): basics/06-communicating-with-host.md docs.rs anchor #tymehtod.env.

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