docs/projects/fmcomms2: add doc for FIR_filter

[BCapota]

PR Description

Added the documentation page for fmcomms2 FIR filter example.

PR Type

• Bug fix (change that fixes an issue)
• New feature (change that adds new functionality)
• Breaking change (has dependencies in other repos or will cause CI to fail)

PR Checklist

• I have followed the code style guidelines
• I have performed a self-review of changes
• I have compiled all hdl projects and libraries affected by this PR
• I have tested in hardware affected projects, at least on relevant boards
• I have commented my code, at least hard-to-understand parts
• I have signed off all commits from this PR
• I have updated the documentation (wiki pages, ReadMe files, Copyright etc)
• I have not introduced new Warnings/Critical Warnings on compilation
• I have added new hdl testbenches or updated existing ones

[CLAassistant]

All committers have signed the CLA.

[IuliaCMoldovan]

Did some minor changes on the text, wrapped it and some rephrasing.
Added links to new documentation.
Added some TODO notes for @BCapota to check and do in another commit.
Afterwards, we can do a rebase on latest main.