Add 10.2.0 changelog and release notes

[cmcfarlen]

Generated CHANGELOG-10.2.0 from the 10.2.0 milestone (650 PRs) and added
the v10.2 sections to whats-new and upgrading.

Authored off 10.2.x rather than master, since master's copies of these docs
describe changes not in this release (e.g. ssl_multicert.yaml).

Discussion needed: #13102

#13102 (Fix connect attempt retries) changes retry limits to be selected by
HostDB state, so DOWN origins are no longer retried at all. I documented it
as a necessary incompatible change, on the grounds that the limits were not
previously applied per origin state as documented.

Two things worth confirming:

• I could not find this on upstream/10.1.x — it still has the old
  two-way is_server_negative_cached() branch and no
  connect_attempts_max_retries_suspect_server record. If it did land there,
  the upgrade note should say so, since operators coming from a late 10.1.x
  would already have the new behavior.
• Whether framing it as a deliberate incompatibility is the right call for a
  release that is otherwise meant to be compatible.

No Sphinx build locally; relying on CI for the docs build.

[Copilot]

Pull request overview

Adds Apache Traffic Server v10.2.0 release documentation by introducing a new top-level changelog file and expanding the existing release-notes docs with v10.2 “what’s new” and upgrade guidance (including the HostDB-state-based origin retry behavior change).

Changes:

• Added a generated CHANGELOG-10.2.0 for the 10.2.0 milestone.
• Added a new “What’s New in ATS v10.2” section to the release notes.
• Added a new “Upgrading to ATS v10.2” section describing incompatible/configuration changes.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
doc/release-notes/whats-new.en.rst	Adds v10.2 “what’s new”, including new features/configuration/metrics/plugins/API highlights.
doc/release-notes/upgrading.en.rst	Adds v10.2-specific upgrade notes, including the HostDB-state-based origin retry behavior change.
CHANGELOG-10.2.0	Adds the generated v10.2.0 changelog content.

Suppressed comments (1)

doc/release-notes/whats-new.en.rst:120

• In the list of new records.yaml settings, this entry is formatted as a literal code span while the surrounding entries use :ts:cv:. Since this is a real config record, it should be :ts:cv: for consistent linking and indexing in the docs.

* :ts:cv:`proxy.config.http.per_client.connection.exempt_list`
* :ts:cv:`proxy.config.proxy_protocol.max_header_size`
* ``proxy.config.quic.server.token_key.filename``
* :ts:cv:`proxy.config.ssl.client.cert_compression.algorithms`
* :ts:cv:`proxy.config.ssl.server.cert_compression.algorithms`

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