docs: reflect that PySTAC writes STAC v1.1.0 by default

[shaikn6]

Related Issue(s):

• doc: clarify STAC version in first bullet list #1513

Description:

What

Update the docs landing-page intro bullet to say PySTAC reads and writes STAC
|stac_version| (currently 1.1.0) instead of the outdated "STAC version 1.0".

Why / evidence

docs/index.rst line 8 still reads:

Reading and writing STAC version 1.0. Future versions will read older versions of
STAC, but always write the latest supported version.

Since v1.12.0 the default write version is 1.1.0
(pystac.version.STACVersion.DEFAULT_STAC_VERSION == "1.1.0", and the 1.12.0
release notes state "STAC v1.1.0 is now the default"). Verified in a clean venv:

$ python -c "import pystac; print(pystac.get_stac_version()); print(pystac.Catalog(id='x', description='d').to_dict()['stac_version'])"
1.1.0
1.1.0

Change

Replace the hard-coded 1.0 with the |stac_version| substitution already
defined in docs/conf.py (rst_epilog) and already used in docs/concepts.rst,
so the bullet tracks DEFAULT_STAC_VERSION automatically. Also tightened the
next sentence ("Future versions will read..." -> "PySTAC will read...") since
older-version reading already works today. One file, docs only.

Verified

• STACVersion.DEFAULT_STAC_VERSION -> 1.1.0
• docutils render of the bullet with the conf.py epilog applied produces
  "Reading and writing STAC version 1.1.0."
• |stac_version| is an existing, working substitution (used in concepts.rst).

Fixes #1513