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

[figueroa1395]

Hello @william-xue, will you continue working on this PR?

Edit: @william-xue Pinging you again since you recently closed your other open PR. This PR doesn't need much additional work, so let us know if you want to finish it, and feel free to PING us for any assistance, we are happy to help.