docs: link LF Energy landing page

[william-xue]

Summary

• Add a link from the documentation landing page to the LF Energy Power Grid Model project page

Validation

• Verified the LF Energy project page returns HTTP 200
• Ran git diff --check
• Checked that docs/index.md contains the new link

Part of #1020

[mgovers]

Hi @william-xue,

Thank you again for your contribution!

After this PR is merged, can you please also do a similar change on the power-grid-model-io repository and power-grid-model-ds repository? It's probably good to homogenize these things as much as possible.

[mgovers]

Either make LFE a first-class citizen

Suggested change

	`power-grid-model` is a library for steady-state distribution power system analysis, hosted by the Linux Foundation Energy (see also the
	[LF Energy landing page](https://lfenergy.org/projects/power-grid-model/)).
	It is distributed for Python and C.
	The core of the library is written in C++.
	Currently, it supports both symmetric and asymmetric calculations for the following calculation types:
	* Power Flow
	* State Estimation
	* Short Circuit
	For conversions from various data format to the power-grid-model, refer to the
	[power-grid-model-io](https://github.com/PowerGridModel/power-grid-model-io) repository.
	For an extended python interface to the the power-grid-model, refer to the
	[power-grid-model-ds](https://github.com/PowerGridModel/power-grid-model-ds) repository.

or first describe the whole business case and then mention LFE

Suggested change

	`power-grid-model` is a library for steady-state distribution power system analysis.
	It is distributed for Python and C.
	The core of the library is written in C++.
	Currently, it supports both symmetric and asymmetric calculations for the following calculation types:
	* Power Flow
	* State Estimation
	* Short Circuit
	For conversions from various data format to the power-grid-model, refer to the
	[power-grid-model-io](https://github.com/PowerGridModel/power-grid-model-io) repository.
	For an extended python interface to the the power-grid-model, refer to the
	[power-grid-model-ds](https://github.com/PowerGridModel/power-grid-model-ds) repository.
	Power Grid Model is an LF Energy project; see the
	[LF Energy landing page](https://lfenergy.org/projects/power-grid-model/).

[Copilot]

Pull request overview

Adds a documentation pointer from the docs landing page to the project’s official LF Energy project page, aligning the documentation with Issue #1020’s request to surface the LF Energy landing page from within the docs.

Changes:

• Add an explicit external link on docs/index.md to the LF Energy “Power Grid Model” project page.

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[mgovers]

Please resolve the open comments + failing CI first