Add FastChem installation step to docs

[timlichtenberg]

Description

Add get_fastchem.sh as a standard installation step (step 9) in the developer installation guide. FastChem is used by AGNI for equilibrium chemistry calculations and was previously missing from the docs.

Changes:

• New step 9 after AGNI installation with cmake/make prerequisites, build commands, and FC_DIR environment variable setup (bash/zsh tabs)
• Renumber subsequent steps (9-14 become 10-15)

Tested locally on macOS (Intel, Sequoia) and confirmed FastChem compiles successfully.

Validation of changes

Ran bash src/get_fastchem.sh inside the AGNI directory on macOS. Build completed successfully, binary produced at expected path. Served docs locally with zensical serve and verified rendering.

Test configuration: macOS Sequoia (Intel) with cmake 4.3.0, AppleClang 17.0.0.

Checklist

• I have followed the contributing guidelines
• My code follows the style guidelines of this project
• I have performed a self-review of my code
• My changes generate no new warnings or errors
• I have checked that the tests still pass on my computer
• I have updated the docs, as appropriate
• I have added tests for these changes, as appropriate
• I have checked that all dependencies have been updated, as required

Relevant people

@nichollsh @ipanagiotou02 @IKisvardai